Is there a strategical reason when composing for the key to commonly be the first and/or last note (or chord) of a song?
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2Other starting notes happen pretty often.– Andy BonnerCommented Feb 18 at 0:49
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2The sense of completion or resolution is so strong, that doing the converse is often used deliberately to unsettle the listener, or for comic effect. (Had to unsplit that infinitive, which has a similar effect.)– Paul_PedantCommented Feb 18 at 10:11
5 Answers
There's no special reason for the first chord, but the last chord relates to the definition of being "in a key". The key note (the tonic) of a key is where the music feels most at rest, and the end of a piece is the most likely place to want the music to feel that way.
Tonal music is defined by the existence of a stable note that centers the music and acts as a kind of home base, called a tonic. In Western tonality, we name scales based on their tonic note. So we call it D major because it has D as its tonic, and any song in that key will center around D.
It's easier for listeners to know what the tonic is if the piece starts with it, and ending with the tonic gives a piece a much stronger sense of completeness and finality. Neither are necessary, though - all you need to do to establish a tonic is emphasize it more than other notes in the piece.
The tonic is, by definition, the 'home' note and the tonic chord is the 'home chord'. Songs typically 'go home'. And, although they don't universally also start there, there's a strong possibility they will.
Some music doesn't have a tonal basis, there isn't a note or chord that 'feels like home'. But this would be quite unusual in a 'song'.
The root chord in any key will feel like home, just as any other will not feel like home. Music is similar to a journey, and when we think about it, a journey will start at home, and it's only really over when we're back at home. There may be stops on the way, (modulations) like any physical journey, but a stop at Aunty Maud's is more like that than 'yes, we're finally home.
I guess as a piece develops most of us get a feel of the key it's in, and even non-musos experience this. Other chords which occur in any piece will have their own relationship with that root/tonic/home chord, but won't be perceived by most as it itself.
A better word for your question might be “tonic”, or the home chord of any given key. Even more often than the first chord, the tonic is usually the last chord in songs. That is because the tonic is home base so you have a sense of resolution when you get there. The way harmony works in general is that chords have different functions and move around from the tonic to and from other types of chords such as the subdominant and dominant. After all this motion the logical place to end is back at the tonic, or home base.