"Yesterday" is a pop song originally recorded by The Beatles for their 1965 album Help!. According to the Guinness Book of Records, "Yesterday" has the most cover versions of any song ever written.
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 (Em 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.