The gap was considered key in determining what Nixon knew about the burglary and what he did to cover up the exploding scandal.
Nixon's main legal risk during 11 hours of questioning near his California home in June 1975 was being caught in a lie. Short of committing perjury, or implicating anyone in his much-diminished cadre of loyalists, he could testify with impunity because a pardon by his successor, Gerald Ford, protected him from prosecution for any past Watergate crimes.
At one confrontational moment, he bristled when pressed for details of a conversation that he said he could not remember. "I don't recall that those specific names were in the discussion," he snapped. "I mean, if you want me to lie about it, I will be glad to."
He added: "Better strike that last."
Historians successfully sued for access to the records. They expected few revelations but were determined to bring to light all facets of that extraordinary episode of presidential disgrace.
Historians certainly did not expect the transcript to solve the mystery of the 181/2-minute gap. Investigators suspected the portion of the June 20, 1972, subpoenaed tape was erased to hide incriminating talk between Nixon and his chief of staff, H.R. Haldeman, three days after the break-in at the Watergate complex.
Nixon stuck to secretary Rose Mary Woods' story that she erased it by mistake, and professed anger when learning how much was missing. Although he said he could not remember what was said during the gap, he had a clear recollection of his aide Alexander Haig telling him that much more was erased than originally thought.
"Rose had thought it was four minutes, or something like that," he testified. "Now the counsel have found that it is 181/2 minutes, and I practically blew my stack."
He said: "If you are interested in my view as to what happened, it is very simple. It is that it was an accident."