Lennon used all three studios at Abbey Road on this night, with EMI staff members and engineers rushing around spooling tape loops onto machines. The session began at 7pm and ended at 3.30am the following morning.
The basis for the track was the second half of the Revolution 1 recording, which appeared in the final mix at various points. Over this, at least 45 sound snippets were added, plus a track featuring Lennon and Harrison speaking random phrases into separate microphones.
Yoko Ono was also present, and can be heard at several points in the track. She contributed a sustained high-pitched note from 6'44"-6'46", and her audio diary, recorded during the first Revolution 1 session on 30 May 1968, was also used in the mix.
The most famous of the tape loops was of an unidentified voice intoning "Number nine" repeatedly. This came from a Royal Academy of Music audio exam, and the sample gave the sound collage its title.
A rough mix was made at the end of the session, which ended with the line "Take this, brother, may it serve you well". What followed on the released version was taken entirely from the latter half of Revolution 1, with the addition of the "Block that kick" sample from an Elektra Records sound effects album.
Revolution 9 was mixed in stereo on 21 June 1968, and edited to a shorter version four days later. The mono version was simply a fold-down copy of this mix, first attempted on 20 August but improved upon six days later.