There was definitely a desire on Nixon's part to have the tapes for his own use in whatever historical work he might do after leaving office, and for reference while he was in office. But the main driving force that led Nixon to approve the use of a taping system was his desire for an accurate record of everything that was said in meetings with foreign guests, government officials and his own staff. He recognised the problem of either intentional or unintentional distortion or misunderstanding and became more and more concerned about the absence of such a record...

In a meeting-between Nixon and me, he decided to install a taping system. It was done in utmost secrecy. Among the President's top advisers, only I knew of its existence. And I soon learned why he insisted on such absolute secrecy. Nixon never intended anyone to hear the tapes except himself. They were never, even to be transcribed.

I remember the day clearly when his attitude towards the tapes was made evident to me. Early in 1972, before Watergate, Alex Butterfield, the White House assistant who maintained custody of the tapes, entered my office. "We're piling up a horrendous amount of tape," he said. "Obviously, nobody is going to sit and listen to all of it. It would take years. So I assume you're going to want it typed up."

He asked me if he should get a crew started transcribing the tapes. "Otherwise, it's going to just build up to an insurmountable task." I raised the question with the President that day. "Do you want to start making transcripts of the tapes?"

He startled me by saying vehemently, "Absolutely not." He didn't want transcripts made at any time. Nobody was, going to listen to those tapes, ever, except himself. Then he added, "I'll never even have Rose Woods listen to them. Rose doesn't know I'm making the tapes. I say things in this office that I don't want even Rose to hear."

At that point Nixon looked up at me sharply as if becoming aware, of what he was saying. "Uh . . . possibly you can hear them, too, but no one else."

Thus the tapes were kept secret even from Rose Mary Woods, until Watergate. And then Rose and I were the first persons to whom Nixon turned regarding the tapes. He asked me to listen and take notes on the March 21, 1973 'troublesome' conference with John Dean - and I ended up with a perjury charge. Rose did little better - ridiculed in the press as she desperately reached for a foot pedal while stretching back for a telephone to demonstrate how 18 minutes of the June 20 tape had been 'accidentally' erased.

Rose would never have had to go through that ordeal if the tapes had not survived as evidence. In the whole story of the White House recording system the one question asked over and over again by both friends and foes of Nixon is "Why didn't he destroy the tapes?" In a telephone call long after I left the White House, and during the period when Nixon was going through the agony of listening to many of the White House tapes prior to releasing the transcripts or turning the tapes over to the prosecutors, Nixon laughed wistfully and said, "You know, it's funny, I was,just listening to one of the early April tapes of a meeting between you and me. I had completely forgotten this, but in that meeting I said to you, 'Bob, maybe we should get rid of all those tapes and just save the national security stuff.' And you said no, you thought we should keep them. Oh, well."

That the CIA should have infiltrated the White House is a startling idea, but McMahon is by no means its only adherent. As H. R. Haldeman has written: "Were there CIA 'plants' in the White House? On July 10, 1975, Chairman Lucien Nedzi of the House of Representatives Intelligence Committee released an Inspector-General's Report in which the CIA admitted there was a 'practice of detailing CIA employees to the White House and various government agencies.' The IG Report revealed there were CIA agents in 'intimate components of the Office of the President.' Domestic CIA plants are bad enough, but in 'intimate components' of the Office of the President?"' Haldeman then goes on to speculate about the identities of the CIA men in the White House. His main suspect is Alexander Butterfield, the former Air Force officer whose White House responsibilities included overall supervision of the presidential taping system. That system consisted of some two dozen room microphones and telephone taps that Wong's Secret Service detachment had installed in the White House and at Camp David; voice-activated by the Presidential Locator System or manually by Butterfield, the microphones and taps fed into a set of concealed Sony tape recorders.' Haldeman's suspicions about Butterfield - who denies that he was a CIA asset-were shared by Rose Mary Woods, President Nixon's personal secretary. Together they criticize Butterfield for voluntarily revealing the existence of the taping system; they point with suspicion to Butterfield's early service as a military aide to GOP nemesis Joseph Califano, and make much of the fact that the circumstances of Butterfield's White House appointment are disputed.

The disputed circumstances concerning Butterfield's appointment are these: both Butterfield and Haldeman insist that it was the other who made the first approach with respect to working at the White House. Butterfield says that Haldeman, a college chum, telephoned him to ask if he would serve as his deputy. Haldeman contradicts this, saying that his call to Butterfield was in response to a letter that Butterfield had written to him, asking for a White House appointment.

Butterfield does not recall having written such a letter. A second element in the dispute is Butterfield's insistence that he had to resign from the Air Force in order to take the job at the White House. Haldeman says that this resignation, which terminated a promising military career, was entirely unnecessary. The suspicion is that the resignation was part of a protocol concerning cover arrangements between the CIA and the Air Force.

Haldeman and Woods are not alone in their suspicions of Butterfield, or in their concern over the Inspector General's report. If Bill McMahon is correct, McCord's seconding of CIA personnel in undercover assignments at the White House amounted to the calculated infiltration of a uniquely sensitive Secret Service unit: the staff responsible for maintaining and servicing the presidential taping system, and for storing its product. Moreover, unless both Haldeman and McMahon are mistaken - about Butterfield's secret allegiance and McCord's loan of personnel to Wong - then the CIA would seem to have had unrivaled access to the President's private conversations and thoughts. Charles Colson, among others, believes that this is precisely what occurred. "The CIA had tapes of everything relating to the White House," Colson told me. "And they destroyed them two days after (Senator Mike) Mansfield asked them to save all of their tapes."

I know that most people think that my inability to explain the 18½ minute gap is the most unbelievable and insulting part of the whole of Watergate. Because of this, I am aware that my treatment of the gap will be looked upon as a touchstone for the candor and credibility of whatever else I write about Watergate. I also know that the only explanations that would readily be accepted are that I erased the tape myself, or that Rose Mary Woods deliberately did so, either on her own initiative or at my direct or indirect request.

But I know I did not do it. And I completely believe Rose when she says that she did not do it. I can only tell the story of the 18½ minute gap as incomplete and unsatisfactory a story as I know it is - from the vantage point of having watched it bring my reputation and my presidency to new lows of public confidence and esteem.

Haig told me that Garment and Buzhardt were completely panicked by the discovery of the 18½ minute gap. They suspected everyone, including Rose, Steve Bull, and me. Suspicion had now invaded the White House. I even wondered if Buzhardt himself could have accidentally erased the portion beyond the five-minute gap Rose thought she might have caused. Using the simple criterion of access to the tapes, there were many possible suspects. Haig and others joked darkly about "sinister forces" being responsible for the gap, but I think that we all wondered about the various Secret Service agents and technicians who had had free daily access to the tapes, and even about the Secret Service agents who had provided Rose with the new but apparently faulty Uher tape recorder just half an hour before she discovered the gap. We even wondered about Alex Butterfield, who had revealed the existence of the tape system. He had had access to all the tapes; in fact, he had regularly listened to random passages to make sure that the system was working properly. But it would have taken a very dedicated believer in conspiracies to accept that someone would have purposely erased 18½ minutes of this particular tape in order to embarrass me.

After an inconclusive public hearing the matter of the gap was turned over to the grand jury and a panel of court-appointed experts. I believe that the story of these experts represents one of the biggest, and least known, scandals connected with Watergate. The White House is not without blame, because we approved the six "experts" appointed by the court. If we had checked, we would have found that they were experts in the theory of acoustics but not the practicalities of tape recorders.

In a January report the experts concluded that the buzzing sounds had been put on the tape "in the process of erasing and re-recording at least five, and perhaps as many as nine, separate and contiguous segments. Hand operation of the keyboard controls on the recorder was involved in starting and again in stopping the recording of each segment." This conclusion was widely reported; not so widely reported was the fact that it came under immediate attack from other scientists and tape specialists, one of whom called it more a "news release than a solid investigation."

One electronics firm pointed out that the apparently incriminating sounds and magnetic marks attributed by the panel to specific manual erasures could have been made accidentally by a malfunction in the machine that caused the internal power supply to sputter on and off. Science magazine reported that other experts agreed with the feasibility of this alternate hypothesis. The only test that the court's experts made of this hypothesis prior to the release of their preliminary report in January was made on a Sony, not a Uher 5000. They apparently failed to see this as a lapse in standard scientific testing procedure.

One of the court's experts testified that in testing the machine Rose had used, they "of necessity had to open up the interior... and tighten down several screws and quite conceivably, for example, may have tightened a ground connection to a point where it was making more firm contact than previously." When they found a defective part they replaced it and threw the defective one away! He acknowledged that after they had worked on the machine it no longer produced the buzzing sound they had noticed beforehand.

After this extraordinary admission, Rose's attorney, Charles Rhyne, said, "So in effect you obliterated the evidence which anyone else would need to test your conclusions, did you not?"

"Yes, in large part," was the answer.

Richard Salmon, national manager of Uher of America, Inc., in Inglewood, California, the manufacturer of the machine Rose had used, scathingly criticized the court experts' report. He said that on some of their specially modified machines pressing the "rewind" button in a certain way would cause the machine to erase automatically.

Nixon told his press secretary, Ron Ziegler, that having reached the low point he was now prepared for the ascent. It was going to be "a turning point for our approach to dealing with Watergate," he later wrote. "`We will take some desperate and strong measure,' I told Ziegler, `and this time there is no margin for error.' " He planned a televised speech for November 7, precisely one year after he'd been reelected, to launch Operation Candor. He would display not the wounded president but the man who had come back from many previous political defeats and who would once more rise from the ashes. The speech would be followed by ten days of "bridge-building" breakfast meetings and private chats with hundreds of Democrats and Republicans in Congress, and a swing through the South to trumpet the message that the president was still on the job and fighting for the country.

This, then, was the setting for one of the more curious episodes in the history of Watergate, the eighteen-and-a-half-minute gap in a taped conversation. The gap has usually been attributed to a mistake on the part of Nixon's personal secretary Rose Mary Woods, and/or to a deliberate attempt by a mechanically clumsy president to erase information detrimental to him. But there was a more sinister aspect to the affair than has previously been understood, and it involves Haig and Buzhardt and an especially well-timed and dramatic revelation by Deep Throat.

Back on September 28, anticipating that the appellate court would rule that the tapes must be turned over, Nixon had asked Haig to arrange for Rose Mary Woods to go to Camp David and transcribe the subpoenaed conversations. Woods was a particularly good choice for this task because she knew intimately the president's patterns of speech, and also knew most of the voices on the recordings-those of Haldeman, Ehrlichman, and other counselors. Fiercely loyal to Nixon, she could be counted on to delete the expletives and the scatological characterizations that sometimes dotted their chatter, not to be shocked by the conversations, and to keep silent about their contents. To help with the technical arrangements, Haig turned to John Bennett, the deputy presidential assistant whom Haig had appointed custodian of the recordings in July.

The next day, Woods and Steve Bull drove to Camp David carrying eight tapes and three Sony tape recorders provided by Bennett. In the privacy of rustic Dogwood Cabin, Woods began what she soon discovered would be a long and painstaking weekend of listening and typing. She spent twenty-nine hours just on the first item listed on the Special Prosecutor's subpoena, the June 20, 1972, meeting in the president's EOB office attended at various times by Nixon, Ehrlichman, and Haldeman, a meeting that lasted from 10:30 A.M. to nearly noon. As pointed out earlier, the quality of the recordings taken from the EOB office was less satisfactory than those recorded in the Oval Office.

The president was at Camp David that weekend and came in to check on his secretary's progress. She told him it was slow going because she had to replay sections of the tape over and over to get an accurate account. Nixon himself put on the headphones and listened for about five minutes. "At first all I could hear was a complete jumble," he recalled in his memoir. "Gradually I could make out a few words, but at times the rattling of a cup or the thump of a hand on the desk would obliterate whole passages." The Oval Office tapes that he had personally listened to back in June had been much easier to understand, he told Woods, and then left the cabin after sympathizing about her arduous task.

Bull had a problem, too, that weekend. He was to locate the conversations called for in Cox's subpoena on the correct six-hour tape reels, and cue them to the proper beginning spots to ready them for Woods. He found the June 20 EOB tape, but could not match up the conversation on the reel with the subpoena list. The list asked for one conversation among the participants, and there had been two on the morning of June 20, one between Nixon and Ehrlichman, and a second immediately thereafter between Nixon and Haldeman.

Haig phoned the cabin on the morning of September 29 to see how the work was going, and Bull told him he simply could not find the one long conversation referred to on the subpoena. Haig called Buzhardt, who had remained in Washington, and explained the situation. Buzhardt made a judgment, which Haig then passed to Woods, who typed a note that she gave to Bull. The note later became part of the documentary evidence assembled by the House Judiciary Committee. It reads, in full: "Cox was a little bit confused in his request re the meeting on June 20th. It says Ehrlichman Haldeman meeting-what he wants is the segment on June 20 from 10:25 to 11:20 with John Ehrlichman alone. Al Haig."

Bull promptly went back to his search, and it was then that he discovered that two of the other subpoenaed conversations were missing; he passed the information to Haig.

The entire crew returned to the White House on Monday, October 1. Woods had still not finished transcribing the first conversation, but back at her White House office she now had a more convenient mechanical setup. The Secret Service had supplied her with a Uher 5000 recorder that included a foot pedal for easy operation.

Just after two that afternoon, she rushed into Nixon's EOB office, visibly upset and saying, "I have made a terrible mistake." After completing her work on the Ehrlichman conversation, she told Nixon, she had forwarded the tape to make sure that she had indeed transcribed all of that section. As she was doing so, a call came in on her office phone and she had a conversation of four or five minutes. When she hung up and went back to work on the tape, she was rudely greeted by a shrill buzzing sound. A section of the Haldeman conversation had been wiped out.

Later, Woods would reconstruct her mistake for a court hearing. She stated that she must have pushed the "record" button on the machine rather than the "stop" button, while unintentionally resting her foot on the pedal throughout her phone call, an action that kept the machine running and, in effect, recording noise over the previously recorded conversation.

Nixon calmed Woods and told her the mistake was not of consequence because Buzhardt had told him that the Haldeman portion was not among the subpoenaed tapes. Haig called Buzhardt, who reconfirmed that the Haldeman conversation was not on Cox's list, and Nixon was relieved.

He should not have rested easy, because Buzhardt was at the very least plain wrong. The counsel had been in continuous touch with Cox since the subpoena had been served, and was in possession of a memo from Cox, dated August 13, that clarified the grand jury subpoena and made it plain that what he expected was Nixon's conversation with "John D. Ehrlichman and H.R. Haldeman in his Old Executive Office Building [OEOB] office on June 20, 1972 from 10:30 a.m. until approximately 12:45 p.m." Any lingering doubt that both conversations were sought was removed by the additional statement in Cox's memo that "Ehrlichman and then Haldeman went to see the President" that morning (italics added for emphasis). Moreover, Buzhardt had also had his alarm bells rung on the matter of the subpoenaed tapes by the news from Steve Bull that two of the conversations couldn't be located. That he reassured Nixon a second time as to the Haldeman conversation's irrelevance suggests that Buzhardt either didn't look at Cox's explanatory August 13 memo, or that he deliberately ignored it. Error of omission or commission?

When Bennett took the stand in Sirica's courtroom on November 6 and described his custodianship of the recordings, his role in providing the tapes to Bull for the trip to Camp David, and so on, the issue was the missing two conversations. The next day, November 7, when Bennett returned to the stand, he told the court that he'd had a talk the previous evening with Rose Mary Woods during which she complained of an unexpected "gap" in one of the tapes she was reviewing for the president.

But this wasn't the gap in the June 20 conversation that she had inadvertently caused. It was a different tape, which as it would turn out had no gap. Woods hadn't mentioned the gap in the June 20 tape to Bennett, but had told Bennett that she'd been reviewing a tape that hadn't even been subpoenaed, an April 16, 1973, Nixon-Dean meeting. "I think she was puzzled," Bennett testified. "The tape was on the machine. She said, `I've got a gap in this.' " Two days earlier, Bennett told the court, he'd given Woods a new batch of six tapes and had said that the president wanted her to listen to that particular Nixon-Dean conversation and that it was among those reels somewhere.

Rose Mary Woods was called to the stand the next day. She said she had checked the tape and had been mistaken and that there was no gap in that tape. When cross-examined, she made clear that all she had meant by the word "gap" was a missing conversation. With that, the inquiry into this particular gap was settled, and the hearing went on to consider other matters. But by raising the specter of one gap, Bennett had opened up the possibility that the still-secret four-to-five-minute erasure on the June 20 Haldeman tape would shortly be uncovered in the court hearing. That, of course, would be damaging both to Woods and to Nixon.

Meanwhile, Bennett's testimony was the occasion for some curious doings at the Washington Post.

There were two stories on the front page of the Post on November 8, 1973, the day on which Woods testified. Under the headline TAPES HAVE PUZZLING "GAP" were two articles. One, under the subhead NIXON AIDE TESTIFIES, was the straight news account of Bennett's court testimony on the previous day, in which he had quoted Rose Mary Woods about a gap that puzzled her.

The second, situated next to the first, was under the subhead PARTS "INAUDIBLE." This second story was written by Bernstein and Woodward, and said that "portions of the seven White House tapes" that Nixon was to turn over to Sirica "are `inaudible' and thus will probably fail to definitively answer questions about Mr. Nixon's role" in Watergate. Quoting "White House sources" to whom the reporters had talked over the past three days, the story said the tapes were marred by "`gaps in conversations,' 'unevenness,' 'excessive background noise,' 'periods of silence,' and 'cut-ins and cut-outs during conversation.' " The article stated flatly that "there is serious concern among the President's aides and advisers that the latest problems regarding the tapes will further strain the credibility of the White House." For instance, the reporters quoted a "high-ranking presidential adviser" as saying, "This town is in such a state that everybody will say, 'They've doctored the tapes.' " This same official had "made clear he rejected that notion."

Two paragraphs down, the reporters quoted a source who clearly did anything but reject the doctoring notion:

"Of five sources who confirmed that difficulties have risen concerning the quality of the tapes, one said the problems "are of a suspicious nature" and "could lead someone to conclude that the tapes have been tampered with." According to this source, conversation on some of the tapes appears to have been erased - either inadvertently or otherwise - or obliterated by the injection of background noise. Such background noise could be the result of either poorly functioning equipment, erasure or purposeful injection, the same source said. The four other sources disputed that there is anything suspicious about the deficiencies and insisted the tapes are marred only by technical problems that can be satisfactorily explained in court."

Who was the one source who believed that an effort might be under way to destroy evidence? Later, in All the President's Men, the authors of the article revealed that it was Deep Throat. Sometime in the first week of November 1973, Woodward initiated a meeting with his source in the underground garage, and received startling information: "Deep Throat's message was short and simple: One or more of the tapes contained deliberate erasures."

One of the enduring images of the Watergate scandal in the 1970s is a photograph of Richard Nixon's long-serving private secretary, Rose Mary Woods, showing how she had accidentally erased 18 mins 30 seconds of one of the crucial clandestine recordings of the president's Oval Office conversations. Three decades later, ranks of audio scientists have been unable to retrieve the conversation that Nixon had with his chief of staff.

Now Ms Woods, who has died aged 87, has taken the secret to the grave. It is fair to say that, once she had re-enacted the incident, few believed that she could have inadvertently pressed the "record" button when a telephone call interrupted her transcription of the tape. Her office layout meant she would have needed the skills of a limbo dancer to achieve what she said had happened.

When the Senate committee investigating the break-in at the Democrats' Watergate office learned that secret recordings had been made of all conversations in the Oval Office, they were immediately subpoenaed. Amid the barrage of legal objections raised by the White House, it was not always clear which tapes were being demanded by the federal court.

One of the first recordings Ms Woods had begun transcribing for the court on November 26 1973 dealt with conversations the president had on June 20 1972 - three days after the burglars had been arrested and their connection with the White House established. That morning Nixon had talked to John Ehrlichman, his domestic adviser, from 10.25am to 11.20am and then to his chief of staff, HR Haldeman, from 11.26am to 12.45pm.

The subpoena for the tapes covered the period from 10.30am to noon, leaving the White House to argue that the Haldeman portion was not relevant. According to Ms Woods's account to Judge John Sirica, she was using a foot pedal to start and stop the tape while she was typing the transcription. When her phone rang she said she had pressed the "stop" button on the machine itself, but then kept her foot on the pedal. After a five-minute conversation she discovered to her horror that she had not only pressed the "record" button but also kept the tape running, deleting a whole section.

Coming from a bright and experienced secretary who had dealt with Dictaphone and other recordings for years, the story never made much sense. It was even less convincing when expert examination showed that there had been at least five, and possibly nine, separate and contiguous erasures of the tape, removing a total of 18 mins 30 secs. According to the Watergate prosecutor, Leon Jaworski, that could only have been done through manual operation of the transcription machine's controls.

Haldeman's own handwritten notes of the conversation, subsequently produced for the court, showed that it had dealt with the break-in and that it was this section which had been erased. Eventually, on August 5 1974 and under enormous legal and political pressure, the White House released the tape of another discussion the president had with Haldeman, on June 23 1972. This revealed the famous smoking gun, showing that the pair planned to use the CIA to cut off the FBI's investigation of the break-in, a conspiracy that brought Nixon's resignation and Haldeman's imprisonment.

Woods was then asked to demonstrate in court how she might have inadvertently made the gap while having a telephone conversation, and her contortions, before a disbelieving court, became known as “the Rose Mary Woods stretch”. She had to create receiving a telephone call while transcribing. It involved removing earphones, stretching with her left hand backward to the main desk telephone, lifting the receiver, then with her right hand pressing the tape record button instead of the stop button, while all the time keeping the pedal depressed with her left foot on its forward play side.

Her desk was 6ft long, and Woods was not tall. From the demonstration it was clear that she could not hold the phone and keep her foot on the pedal inadvertently, as she claimed, but would need to keep her leg fully stretched throughout a long phone conversation to perform the operation.

At one stage it looked as if she might have to join a long line of White House employees charged with obstructing justice. But she always insisted the gap was accidental, and although the prosecution said her account was implausible they were not able to disprove it.

Ms. Woods, who worked for Mr. Nixon for decades and joined him in exile in California after his 1974 resignation as president, took part of the blame for the missing portion of a taped conversation between President Nixon and the White House chief of staff, H. R. Haldeman, on June 20, 1972, three days after the break-in at Democratic headquarters in Washington.

In one of the most memorable photographs of the era, Ms. Woods is shown trying to recreate the chain of events in which, she said, she could have accidentally erased part of the tape as she was transcribing it on Mr. Nixon's orders in 1973, after the scandal broke. The photograph shows Ms. Woods at a desk, reaching far back over her left shoulder for a telephone as her foot hits a pedal controlling the transcription machine.

"I am most dreadfully sorry," she said in court testimony in November 1973 in explaining that through some "terrible mistake," she had pressed the wrong button on the pedal and recorded over the tape. She said that she had immediately notified Mr. Nixon of the erasure and that he had assured her that "there's no problem because that's not one of the subpoenaed tapes."

Still, Ms. Woods testified that her error might explain only about five minutes of the gap, not the full 18½ minutes. In 2003, the National Archives said a panel of audio specialists had analyzed the tape and been unable to recapture the lost conversation.

News that so much of the tape had been deleted eroded Mr. Nixon's credibility on Capitol Hill and with the Watergate special prosecutor's office at a time when his presidency was beginning to unravel. The gap consisted of a buzzing sound that obliterated part of a conversation in which Mr. Nixon was instructing Mr. Haldeman to take "public relations" moves to divert attention from the break-in at the Watergate office complex.

She had worked for Nixon since 1951 and was so close to the family that Tricia and Julie Nixon called her "Aunt Rose," and she swapped clothes with first lady Pat Nixon. Nixon wrote in his memoirs that he asked his secretary to tell his wife and daughters that he planned to resign Aug. 9, 1974.

But she was far more than a family friend. Long before Haldeman and domestic aide John Ehrlichman joined the presidential campaign, Miss Woods was Nixon's gatekeeper. Reporters said she controlled who could see her boss -- and punished those she deemed critical.

"Rose would die for (Nixon)," Beatrice Lucille Miller, her first boss, told The Washington Post in 1974. "Rose would just lay down her life and die for him."

She remained loyal even after Nixon left office, keeping a sort of shrine to him in his hideaway office in the Executive Office Building until the Ford administration made her vacate it, according to an item in McCall's magazine in 1975. "Rose left his half-smoked cigar in the ashtray, his glasses on his desk and his wastebasket half-filled," the article said.