In the Beatles’ first ever studio meeting with the producer George Martin, in 1962, Martin asked the band whether they had any issues with the session. “Well, there's your tie, for a start,” George Harrison replied. That wisecrack, the story goes, kicked off a bantering, joking rapport between the young Liverpudlians and their record-label overseer—which in turn kicked off one of the most fruitful band/producer relationships in rock history.

That early anecdote says a lot about what’s so remarkable about George Martin—who died yesterday at the age of 90—and the Beatles. Martin was born in 1926, and had put in time working on classical, jazz, vocal pop, and comedy records before signing the band. For most of his years working with them, he served as the clean-cut elder adviser to the mop-top avatars of counterculture; tellingly, Martin always called them “the boys.” Though he’s often and justifiably called the Fifth Beatle, his outsider status was part of why he was important—the way he pushed the band to bridge their rock energy with wider, older traditions while pushing the possibilities of the studio.

Not that he wasn’t already forward-thinking before meeting the Beatles. His New York Times obituary quotes a 2003 interview where Martin said that when he first joined EMI, “the criterion by which recordings were judged was their faithfulness to the original. If you made a recording that was so good that you couldn’t tell the difference between the recording and the actual performance, that was the acme. And I questioned that. I thought, O.K., we’re all taking photographs of an existing event. But we don’t have to make a photograph; we can paint. And that prompted me to experiment.”

That experimentation took the form of collaging and manipulation, techniques he would later teach the Beatles (Lennon in particular became obsessed with the trick of playing tapes backwards, a concept Martin said he introduced to him). The Times also notes that when the band played him early songs like “Love Me Do” and “P.S. I Love You,” he pushed them to polish the vocal harmonies and fine details of the arrangements in order to elevate tunes that otherwise might be seen as derivative—a mentality that obviously guided their career going forward.

Paul McCartney has written a remembrance of Martin that includes the story of recording “Yesterday,” a breakthrough for the band in terms of expanding the notion of what their sound could be—and in turn, what their genre could be. The leap forward came about because Martin encouraged McCartney to pull from outside rock to create something original, while also drawing from his own technical prowess to execute:

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." […] 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.

Martin’s mix of adventuresome spirit and classic chops proved essential time and again, whether engineering the tuneful scuzz of “Revolution” or crafting the aural voyage of “A Day in the Life.” In the latter song, he guided a dubious 40-piece orchestra to create the song’s cacophonous, atonal swell by writing out sheet music for the players.

Martin would continue to prove his versatility and studio genius in the decades after the Beatles broke up, producing for artists ranging from Cheap Trick to Celine Dion. But perhaps the most perfect example of his contribution to music history is the Beatles’ “Strawberry Fields Forever.” The band recorded two versions of the song: a straight-ahead rock track and a dreamy orchestral piece. When told that they wanted to combine the different takes into one work, Martin objected that they were at two different tempos and keys. “"You can do something about it," John Lennon told him. "You can fix it." And Martin did, finding a way to make two familiar sounds to collide into something new, for neither his first nor last time.