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Just by curiosity. What is the minimum of notes and chords i need to determine the key of a song?

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If you can find the 2 half-note steps you can find out the key. –  Caleb Jul 17 at 2:34
    
@Caleb - so that's 4 notes minimum? –  Tim Jul 17 at 6:56
    
@Tim Yes. It's what I usually do if I'm trying to play by ear. –  Caleb Jul 18 at 0:11

4 Answers 4

There isn't any hard and fast rule. The first thing is that the key signature narrows it down to two keys. So, for example, if there are no sharps or flats in the key signature, the key is either C major or A minor. Most of the time, the first few measures in the piece will establish whether you're in the major or minor key.

Beethoven's 5th symphony is a famous example of an intentionally ambiguous opening. There are three flats in the key signature, so it's either Eb major or C minor, but it's impossible to tell which from the first four bars. The first four notes could be either the top two notes of a Cm chord or the bottom two notes of an Eb chord. The next four notes are no help; they could be either the middle two notes of a Bb7 chord (and therefore the V7 chord in Eb) or the top two notes of the G7 chord (and therefore the V7 chord in Cm). Then, the next four notes end on a Cm chord, leading up to a strong cadence on the G7 chord. This establishes the piece firmly in C minor. So sometimes, you have to dig into the piece a bit before you know.

One way to tell whether a piece (at least in classical music) is in the major or minor key associated with the key signature is to look for an accidental that raises the 7th note of the scale. That's usually a dead giveaway that the piece is in the minor key. This accidental will be a natural in keys with three or more flats, a double-sharp in keys with five or more sharps, and a sharp in every other key.

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that's not fair !! OP never mentioned key sigs!!! Obviously with that as a given the choice is 2 - unless someone's written modally, properly... –  Tim Jul 16 at 18:30
    
A song's key signature doesn't always relate to its tonality. If Arthur's Theme ("Best that you can do") and "El Shaddai" are transposed so that the first chord is Dm, both would start with the chord sequence "Dm G7 C F Bb E7..."; and both melodies have the same tonality (one could sing them simultaneously and they'd fit) but the key signature of the former would be A major (I think) and the latter, C major. –  supercat Jul 16 at 18:40
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@supercat - Arthur's theme modulates like crazy. Whatever key it may be written in, there's going to be accidentals everywhere.Then it effectively goes into another key. El Shaddai really stays in the same key all through.I've played stuff where I've asked at the end "Why was it written in that key?" as a different one would have been easier to write, and read. –  Tim Jul 16 at 20:24
    
@Tim, if it's in a weird key, it's usually the singer's fault. :-) I have one song I sing in Cm. The fiddler hates soloing on it. –  Kevin Johnsrude Jul 16 at 22:22
    
@KevinJohnsrude - it's a perennial nightmare with singers insisting that they can only sing a particular song in one key. Tessitura often says they're wrong !But my beef was that you could,for instance, write a song in, say, F, with a Bb, and naturalise any subsequent Bbs in the piece. So, in fact, it wouldn't really be in F.Or maybe there aren't any Bbs in the piece.Why? gets asked : the answer is rarely forthcoming.If a piece modulates a lot, strictly, it should be written in the key it'll end in,but it isn't always.Hence,accidentals everywhere, which would happen anyway,so why not minimise? –  Tim Jul 17 at 6:54

It depends on how many different chords there are in the song. For jazz, the traditions of chord substitution, use of ii-V progressions, possible polytonality and shifting key centers may mean you have to look at a dozen chords and grok the chord movement to understand the key.

But for the overwhelming majority of songs you only need one chord, the final one. The last chord almost always resolves to the tonic.

As for the number of notes, I have a hunch that the overwhelming majority of songs end on a half or full bar of the tonic.

So let's assume that we're listening to a tune and there's no polytonality and no shifting of key centers and no modality. If the time signature is 4/4 then to determine the key by the notes I would either

  • collect all the notes that play on the 1 and 3 of each bar and eliminate the obvious passing tones or
  • collect all the notes that appear to be chord tones: This is difficult for me to explain but easy to do. I look for notes that fit one of these combinations: 135, 351, 513 where 3 could be natural or flatted. In jazz it's a little more complicated but you get the idea.

This collection of notes will give me either the full scale or the strong notes. The strong notes for major keys are usually the pentatonic major scale, and for minor keys usually the pentatonic minor. With these notes plus the tonic in the last bar I've got the key.

So if we ignore the note harvesting and assume no modality, it's 2 notes: the 1 and the 3. If our collection of notes indicates either modality or an exotic scale then it could be up to 7 notes.

If we include the note harvesting then for a 32 bar song it's somewhere between 5 notes (chord tones) and 64 notes (1&3).

Although honestly, if I was listening to a song and wanted to find the key, I'd comp along until I gathered all the skeleton chords.

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What's the difference between a blues band and a jazz band? A blues band plays 3 chords in front of 100 people, and a jazz band plays 100 chords in front of 3 people. –  Bradd Szonye Jul 16 at 22:06
    
(Good answer, by the way, I just couldn't resist making the joke!) –  Bradd Szonye Jul 16 at 22:14

Chords, I'd say two. One will be the tonic, and the other, usually, the dominant. There are songs that use tonic and sub-dominant, which, funnily enough, is the same relationship, but the other way round.Given a minor key, the dominant may well be major, so it's easier to determine.Obviously, the more the merrier.Working through a song, three could be enough to be accurate, but start at the end chord, which OFTEN is the tonic, you maybe don't need more than just the one ! This could turn out to be not the simple question it appears.

Note-wise, hope someone else will reason that out.Given some underlying harmony, there will be a simple answer,but I wonder whether you mean notes/chords separately or together...

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95% of the time the first chord will tell you what key the song is in

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Did a test this week on 100 songs at random. 40% were started on the key chord (not including anacruces).95% probably represents the LAST chord. –  Tim Jul 16 at 18:28
    
@tim: I think this may vary strongly with the kind of music you're into. The figure in my collection is 72%, but 95%, while clearly an estimate, does sound plausible. –  Marcks Thomas Jul 16 at 20:21
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@MarcksThomas - I grabbed a busker's book with 101 numbers in it, at random.There's certainly less chance of a piece starting on tonic than finishing on it. –  Tim Jul 16 at 20:27

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