Last night, news broke of the tragic passing of legendary producer George Martin. Affectionately known as the “Fifth Beatle,” Martin was the one who signed The Beatles to label when all other major British labels had refused the Fab Four. In response to the sad news, Sir Paul McCartney has written a touching letter to pay tribute to his friend, shared via Facebook.

“I’m so sad to hear the news of the passing of dear George Martin,” McCartney begins. “I have so many wonderful memories of this great man that will be with me forever. He was a true gentleman and like a second father to me.” Discussing his close relationship with Martin and his gratefulness for how he had signed The Beatles, Paul also shared his favorite memory with George Martin:

It’s hard to choose favourite memories of my time with George, there are so many but one that comes to mind was the time I brought the song ‘Yesterday’ to a recording session and the guys in the band suggested that I sang it solo and accompany myself on guitar. After I had done this George Martin said to me, “Paul I have an idea of putting a string quartet on the record”. I said, “Oh no George, we are a rock and roll band and I don’t think it’s a good idea”. With the gentle bedside manner of a great producer he said to me, “Let us try it and if it doesn’t work we won’t use it and we’ll go with your solo version”. I agreed to this and went round to his house the next day to work on the arrangement.

He took my chords that I showed him and spread the notes out across the piano, putting the cello in the low octave and the first violin in a high octave and gave me my first lesson in how strings were voiced for a quartet. When we recorded the string quartet at Abbey Road, it was so thrilling to know his idea was so correct that I went round telling people about it for weeks. His idea obviously worked because the song subsequently became one of the most recorded songs ever with versions by Frank Sinatra, Elvis Presley, Ray Charles, Marvin Gaye and thousands more.

George Martin passed away yesterday evening at the age of 90. Ringo Starr was the first to confirm of the news via Twitter. Read Paul McCartney’s tribute to Martin in full here, and listen to The Beatles’ 1965 classic, “Yesterday,” below.