What Beatles song changed their trajectory the most?

"Help!" was a change lyrically although we did not know it at the time. It was one of the first songs that was on a personal level. Also on Rubber Soul, "Run For Your Life" was a different take on love.Threatening the girl was a new direction.

I don't know shite.

My answer is always A day in The Life

I'd go with You've Got To Hide Your Love Away.

Amazing song writing inspired by a bit of Dylan and a lot of weed.

Lots of really good answers to this, because they changed their musical trajectory (and by extension, the trajectory of popular music in general) multiple times over their 7 years as the biggest rock and roll band in history.

However, rather than offering an answer to the OP, I figured I'd share my favorite Beatlephile site: Alan Pollack's Notes On The Beatles

Here, for example, are some excerpts from the author's analysis of "Norwegian Wood":

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This understated and characteristically oblique song of John's is also admirably economical in terms of both form and content, with everything but the bridges being derived from the same hook motif. In lesser hands, the large amount of unvaried repetition and static harmony of this piece might have resulted in a moribound, boring mess. The Beatles leverage it all in favor of and unity and focus...

The much commented-upon use of a sitar was surely ground breaking enough at the time per se. What I am particularly struck by, given the novelty-numbing distance of time, is the extent to which the psychadelic buzzing of that exotic instrument is so uncannily complemented here by the high level of percussive noise achieved by using a hard pick on the otherwise standard 12-string guitar...

The hook phrase stretches out liesurely over eight measures that are bound to an harmonic "envelope" on the I chord (E). We could likely argue all night about whether or not one hears equally implicit chord changes during this hook, but we've got better things to do all night than that, right, buddy ? In any event, this drone-like element in the harmony combines with the sound of the Indian sitar to create a stylistic "sound" which, if you stop to think about it, anticipates here in "a John song" what would soon become very much a specifically Harrisonian trademark...

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...and so on.

Yeah, it's wordy, and possibly way overthought, but I find it all very fascinating. Well worth perusing if you are truly a fan of the Fab Four.

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My answer is always A day in The Life

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This would have been my answer. Didn't this influence Brian Wilson to create Pet Sounds or maybe vice versa.

Tomorrow Never Knows

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Tomorrow Never Knows

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Came here to post this.

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I think “Tomorrow Never Knows”, “I Want To Tell You”, or really the entire Revolver album in general, is the answer.

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Disagree.

Rubber soul is when they decided to make brilliant albums and not just singles.

The tour of the usa right before rubber soul is when they met dylan and smoked weed and started taking lsd.

That was the turning point in their career.

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What Beatles song changed their trajectory the most?

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At least for George I'm going with 'Something'.

Despite him coming up with different layers, textures, and modes, that defined their sound throughout the years he was always 3rd fiddle. With Something that changed in 69 as he stepped up and showed he had what he took to write music with real lasting appeal. He was stepping out not only with writing but with other musicians as well. While Paul and John continued dueling or feuding, George was collaborating with other leading musicians and it was paying off in spades. For example writing and recording 'Badge' with Clapton. All the while he had amassed enough to do a well written triple LP 'All Things Must Pass', which also featured a bunch of really good musicians and pushed him to the front as my fav of the FAB4.