Here is how it usually worked at the Abbey Road studios: producer George Martin would be up in the control room while the Beatles were down on the studio floor.
But on April 28, 1966, Paul McCartney and John Lennon were the ones in the control room. George Martin was down on the studio floor with eight session musicians.
A double string quartet.
On that day, they recorded the instrumental tracks for “Eleanor Rigby” (as scored and conducted by Martin). Paul would cut his vocals the next day.
To hear the strings-only mix of “Eleanor Rigby,” click here. This track is available on the double-CD “The Beatles Anthology 2.”