Music and lyrics

The tonic key of the song is F major (although, since McCartney tuned his guitar down a whole step, he was playing the chords as if it were in G), where the song begins before veering off into the relative minor key of D minor. It is this frequent use of the minor, and the ii-V7 chord progression (Em7 and A7 chords in this case) leading into it, that gives the song its melancholy aura. The A7 chord is an example of a secondary dominant, specifically a V/vi chord. The G7 chord in the bridge is another secondary dominant, in this case a V/V chord, but rather than resolve it to the expected chord, as with the A7 to Dm in the verse, McCartney instead follows it with the IV chord, a Bb. This motion creates a descending chromatic line of C B Bb A to accompany the title lyric.

The string arrangement supplements the song's air of sadness, especially in the groaning cello melody that connects the two halves of the bridge (on the line, "I don't know / she wouldn't say") as well as the descending line by the viola that segues the chorus back into the verses. This simple idea is so striking, McCartney mimics it with his vocal on the second pass of the chorus. This viola line and the high A sustained by the violin over the final verse are the only elements of the string arrangement attributable to McCartney rather than George Martin.

An unusual aspect of this song is that its first phrase is only seven measures instead of the customary eight measures found in most all popular music. The use of the IV to I cadence (called a plagal or "Amen" cadence) at the end of this first phrase results in the seven measure phrase sounding perfectly natural and almost religious in nature.

McCartney's eventual lyric for the song was sombre and fitted the reflective melody. Although the lyric is rather vague, it could be interpreted to reveal sadness about a lost loved one. Most believe that it revealed sadness about a breakup, while some believe that there is an obscure connection to this song with the death of his mother when he was a teenager. When the song was written, the lyric was considered unusual for a McCartney composition. Until then, most of McCartney's songs had been positive and upbeat; Lennon was considered the Beatle with the more introspective and sad lyrics.

When the song was performed on The Ed Sullivan Show, it was done in the above-mentioned key of F, with McCartney as the only Beatle to perform, and the studio orchestra providing the string accompaniment. However, all of The Beatles played in a G-major version which was used in the Tokyo concerts during their 1966 tours.

When McCartney appeared on "The Howard Stern Show", he stated that he owns the original lyrics to "Yesterday" written on the back of an envelope.

In July 2003, British musicologists stumbled upon similarities between the lyric and rhyming schemes of "Yesterday" and Nat King Cole's "Answer Me", leading to speculation that McCartney had been influenced by the song. McCartney's publicists denied any resemblance between "Answer Me" and "Yesterday". From the story:

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["Yesterday"] begins with the line: "Yesterday, all my troubles seemed so far away/Now I need a place to hide away". Answer Me has the line: "Yesterday, I believed that love was here to stay, won't you tell me where I've gone astray."

”

Ian Hammond speculated that McCartney subconsciously based "Yesterday" on Ray Charles' version of "Georgia On My Mind," but closed his article by saying that despite the similarities"Yesterday" is a "completely original and individual [work]."

In 2006, Italian producer and songwriter Lilli Greco claimed "Yesterday" to be a cover of a 19th century Neapolitan song called Piccere' Che Vene a Dicere'. In the same article, Greco claimed that McCartney and Lennon had an "encyclopaedic knowledge" of world music and were particularly fond of Neapolitan songs. Neither Lennon nor McCartney could read sheet music, and the article does not indicate when—or if—McCartney or Lennon heard a performance of the song.