What is the method to figure out the key to someone else's song?
Let's assume you've never played the song on an instrument and it's not your own song.
Is it faster to try to figure out chords and then guess and surmise the key, or perhaps a similar method but with a basic scale or figuring out the melody by ear?
(So far it seems to me that the fastest non-instrument way is to Google the song's sheet music but this question is about using your ear and instrument to find the key.)
Do you find it faster to try to figure out chords and then guess and surmise the key? ... or perhaps a similar method but with a basic scale or figuring out the melody by ear.
I would personally go with none of the above!
The "key" of a song (or composition, etc.) is just another word for the "tonic" of the piece. And by tonic, we mean the most stable pitch: the pitch to which all other tension points and resolves. The common metaphors here are that tonic is "home base," and that it's the sun around which all other pitches (the planets) revolve and to which they are all gravitationally attracted.
As such, if you're looking for the key of a piece, find the pitch that sounds the most stable. This will be the tonic pitch.
If you want to get more specific than that (say, C major or C dorian), then you'll need to go the next step of taking a pitch inventory and seeing what scale collection it creates.
But honestly, so much music you hear these days doesn't stick to a single scale; in many (most?) cases, you're better off finding tonic and not trying to extend your key categorization any further than that.
By ear
This is an ear training approach that many music programs use: try to sing the tonic note of whatever chord progression you are listening to. You start by singing the tonic of chords, then move to sing the tonic of a progression, to eventually sing the tonic of a song. That's your key. By doing this you are trying to find the pitch that sounds more stable, using only your ear.
It's much easier if you have some ear training background though. Stuff like identifying intervals and simple chords.
Using this idea, and your instrument, without the ear training part (you'd be training your ear, just not as much), you can try to find the bass notes of the chord progressions, or a bass note that kind of works through all the progression.
By harmony
Another way, where you need to be somewhat familiar with harmony, and know some of the chords of the song, is by looking at the quality of the chords involved. C mayor scale, for example, builds the chords C major, D minor, E minor, F major, G major, A minor, and B diminished. if you have a F major - G major - C major progression, you are probably in C major.
That's with triads, but with dominants it can be even more explicit. Search for dominant 7 chords and other chords that play that function (like diminished 7 chords), those often resolve to the tonic. There's secondary dominant chords that do not resolve to the tonic though, but they normally resolve to something important like the dominant or subdominant.
Most of the time you can get away with paying attention only to the cadences (V I, IV I, II V I, etc), since they normally paint a pretty good picture of the tonality.
By scales and melody
Sometimes when learning new songs, I try to play some important parts of the melody on my guitar, and then improvise a little over that idea. At that point the key becomes apparent most of the time, since the key is based on scales. If you can play F G A Bb C D E (doesn't need to be that order) and it sounds cool, you are probably in F major or D minor or any of the parallel keys.
For the three approaches, remember that harmony can get tricky. For the minors we have common variations like harmonic, melodic, and dorian. So it's very common that the 6ths and 7ths are "displaced", along with some of the harmony. There's also key modulation, and music that lacks tonality and key!
But for the less exotic songs, which are the huge majority, you can keep it simple.
I don't agree with the answers provided by Von Huffman and Viking Children, agree with Richard's.
The tonic note is literally key to it all. Find that, and you usually have the scale used and the chords available. may be major, may be minor, but that's not a big deal to work out.
The tonic is the 'home' note, and thence the home chord. It's the one where in a piece, things could stop at that point. It's usually the chord that occurs most often, and the one the piece gravitates to at the end. Out of the 3 main chords in a major piece, there's I, IV and V. Since IV and V are so close together - a tone apart, it's not too bad to hear when they're played one after the other. IV>I seems to go down, and V>I seems to go up. Playing them in all different keys will get the ears more used to how they sound following each other.
Playing in the house band at open mics, this works well, and the key's found within 3 or 4 bars, normally. Useful when someone can't tell you beforehand, or even sometimes tells you a key that it really isn't.
For me, listening to the bass usually reveals it best. I'm listening out for notes 1, 4 and 5. If 1 and 5 occur in the same bar, it feels very stable, so I'm listening for when it comes again. It'll often be at the end of a verse, and the end of a middle eight will be a V chord, most likely to start a new verse on the I. Ha, found it again!
I play guitar and generally start with ear and instinct. It gets a lot easier when my conscious mind knows what key signature I'm in. In order to confirm to my conscious mind, I look for the semitone intervals that match the song. For example, if the notes B and C both sound right to me, then it's either a mode of C major (with C as the tonic) or a mode of G major (with C as the fourth.)
I just figure out the scale, and from there you're going to be able to ballpark what key you're in. A key can use different scales, but the tonic note is what gives the key away. If it's an A major scale chances are you're playing in A. It gets mroe complicated in classical or jazz, but for your rock/pop/folk/country/blues music this works more often than not.
The way I do it, as the song plays, I just play random notes on my instrument. It's very apparent which notes are not in the key as they will sound like a train wreck when you play them along with the song. So I quickly know what notes to avoid, the notes that mostly sound like they "fit" will be the seven notes in the key.
The note that they all tend to resolve, especially at the end of a phrase, and definitely at the end of the song, will be the tonic of the key. So you can just fast-forward the song to the end and pay attention to the last note of the melody. For example, in this song from Wizard of Oz, If I Only Had a Brain, if you fast-forward to the end of it you'll see that the last note when Scarecrow says "Brain" at 3:44 is a D, so the key is in D. And it's a D major scale, with the notes D, E, F♯, G, A, B, and C♯. If you try any other notes you'll notice they don't fit if you play them as the song is playing.
Also note that on a piano, when you find the black/white notes that "fit", just based on their visual pattern you'll know the scale, assuming you've practiced scales. The more you do this the faster you get at it. Just try this on any random song you listen to.
It's obvious to me that there are several different methods for determining the key of a song. I use the method described by Richard and I find the key almost immediately most of the time, but just like everything else in music, I had to work at it in the beginning, I had to learn how to hear it. I don't think that one size fits all with this technique and it's up to the individual to find the method that works best for themselves. Also, I'm not absolutely certain any method is completely fool proof every time.
