If you had to pick a single word to describe the song "Yesterday", it would likely be "nostalgic". And while admitting that how music makes a person feel is extremely subjective and thus any attempt to perfectly and uniformly explain anything emotional is inherently doomed to failure, there is a degree of benefit to be had from attempting to understand why certain combinations of words and musical tones prompt certain emotional responses.
With that caveat in mind, both the music and the lyrics of "Yesterday" provide a strong nostalgic feeling in the majority of listeners. With words it is relatively easy to accomplish that ambiance - through lyrics like "Yesterday all my troubles seemed so far away, now it looks as though they're here to stay, oh I believe in yesterday" and "I said something wrong, now I long for yesterday"; but what about with the music? Why does this music have a similar emotional content? What is it about this music that elicits this reaction?
There is a particular conflict in the lyrics, a discrepancy between actual reality and what the singer wishes was reality - and what was reality in the past: "Suddenly, I'm not half the man I used to be" and "Yesterday, love was such an easy game to play, Now I need a place to hide away". Reflecting that conflict, the music employs a conflict of its own regarding the tone B: is it B-natural or is it B-flat?
To illustrate, observe the example below (click it to enlarge). In the graphic, notes and chords using B-natural are light blue, while those using B-flat are red.
To illustrate sonically how this could have been different, here are a few MIDI examples:
The use of B-naturals slightly brightens the music (especially right after the word "believe") providing a brief respite from the gloom. The use of B-flats, by contrast, makes it slightly darker and more melancholy. But the combination of both - the use of B-flats and B-naturals side by side - helps elicit that potent twinge of nostalgia that "Yesterday" is so famous for, musically paralleling and symbolizing the singer's own nostalgic feelings regarding his past and present. In this way, Paul McCartney solves the problem of "How can the music and lyrics work together to both elicit the same nostalgic feeling?"
This blog is a workshop for developing my analyses of The Beatles' music.