How do you decide which key signature a certain music is in by ear, from hearing it?

Question

When I hear a song or a melodious tune, I can often figure out the relative pitches and play the tune on the piano. But I usually only play it on C major (the simplest scale for me--occasionally G major , because I often can't decide between "mi fa" and "si do" (E-F versus B-C). So if I were to write its score, I would write it in C major.

My question is: what about professional musicians? If they are to jolt down the score or play by ear when hearing a new music piece, how do they know or decide what key signature to write it in? Why would one write it in any one other than the simple C major?

Example 1: from hearing the first few notes (first 3 bars?) of the last movement of Cesar Franck's Violin Sonata (the violin part), my immediate reaction is either:

(a) "so, la, fa, mi-re, do", and play it on "G, A, F, E-D, C";

or alternatively,

(b) "re, mi, do, si-la, so", and play it on "D, E, C, B-A, G"

But I guess neither is the original score.

Example 2: Auld Lang Syne:

I always thought the score were: (c) "so, do...do, do, mi, re...do-re, mi, do...do, mi, so, la---" = G C...C C E D...C-D E C...C E G A---

But I just read the score is actually: (d) "do, fa...fa, fa, la, so...fa--so, la, fa...fa, la, do, re---" = C F...F F A G...F-G A F...F A C D---

(Upon reflection, the keys in (d) are exactly the solfege in (c) but uttered in F major.)

Answer 1

First, scales are not keys. they're certainly related, but we don't write in scales, we write in keys - using scale notes, usually, but not always.

Second, your other key is more likely F, as G has F♯ (fa diese).

Third, using key C is not a problem on piano, and a lot of players would be happy there. But it's not always going to be piano. It could be vocal, and then some notes in key C would be out of reach. It could be trumpet or clarinet, where music is actually written in a different key from the sounds that come out - they're transposing instruments. It could be that you want to play along with a recording of the piece, in which case, you'd better play it in the same key!

Those 'blessed' with absolute pitch would probably write out in the correct key, as they know what the pitches are.

Not knowing the key of a piece, pro musos would be able to write it out in whatever key, and it wouldn't matter to them. Although to save time later, they would probably establish an appropriate key before they start.

Answer 2

From what you have given us there:

(b) "re, mi, do, si-la, so", and play it on "D, E, C, B-A, G"

is correct to the notated score, just transposed from A (the key of the movement) to C.

This is totally valid in terms of learning a melody, but it does remove it from its context, its key signature.

Having done a lot of melodic transcription, from a jazz background, it is a lot easier if you have the key signature before working out the melody, and luckily for me, most standard jazz tunes have fairly similar chord changes.

For other times when I have been transcribing, I will often transcribe the melody exactly as it is played, and then afterwards go back and transcribe the chord changes, and from there I find it easier to determine a key signature, especially if in the case of jazz, the key centre may change multiple times per song.

I cannot speak for the classical realm though, as I have been outside of it for many years, but I have yet to come across a classical pianist or violinist, for example, who will transcribe the chord changes to say, Chopin or Bach, which I think is a shame.

So, in answer to your question, I believe, context matters, and it is worth trying to transcribe music exactly as it is which will give you more information about the piece as a whole.

Answer 3

I know a fair number of professional musicians, and I'm one myself. The circles that I work in don't really have one specific method to determine the key of a song if its not already written out in sheet music form. My own method for duplicating the key of a song is to listen to the song and play along with it as it's playing. I'm able to usually find the key of the song in a measure or two if I'm already familiar with the song, and then I can use my ear training and relative pitch to determine the melody of the song and can write it out if needs be. I should say right here that I can usually hear in my head how it will sound before I ever put pen to paper. Others most likely do it differently.