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Lately I've been noodling around on the piano, picking out melodies with my right hand and trying, by trial and error, to match up chords with the left hand. If I could instantly recognise the key the melody is in I could figure out the chords easily, from a chord chart perhaps. My question is, is there an easy way to recognise the key, for example, how do you know if the melody your playing with the right hand is in the key of C major or A minor? Probably a simple theory problem, not my strongest subject I'm afraid.

I don't know if I can explain this very good, but I will try. I am kind of behind on theory because I didn't pay much attention in class.


First you need to learn how to build the scales in those keys and know how a major key is different than a minor key not just in sound but in structure.

If the melody is in C major it will usually start on C in the beginning and it will more than likely end in C at the end of the piece or phrase. In A minor it will likely begin on A or go back to A often as it's the tonal center of the key. Both C and A minor have no signs in their key signature, so if you are determining the key of a whole piece and it has no signs then look at the beginning and look at the end. Does it end with A or C, etc... The problem is, it's not always the root note that it starts or ends on, you'd need to learn harmony and how to build the triads.

Then you need to know what sharps or flats the most used keys have.

When you look at a particular passage of music, check the accidentals that will occur, new flats or sharps, naturals, etc and figure out which key they belong to. New sharps or flats don't always mean a new key. Minor keys are a good example of that. We often have C# in D minor (the 7th degree in minor is often raised, if it is a key with flats it will be a "natural" which would also raise the note).


Here is what you can do. Look for the most important notes in a bar and try to use chords that have notes notes in them. You will be able to judge if they are right or not. Note of longer duration for example might be more important than short duration notes that might just be leading the melody to somewhere else in between but the chord you use before them will sound good together with them as well.

You don't need to put a chord to every single note. Also look at the shape of the notes in the one bar, put them together horizontally in your mind, do they make up a chord that you might recognize? This is a giveaway for the key it's in currently and it happens often.

I will give you an example with a piece I am playing these days.



This is Bach's G minor sinfonia. It's a polyphonic piece (meaning all voices are of equal importance and it's not simple melody and accompaniment). Look at the shape of the notes in that measure (left hand), put them together. The first 5 notes played together spell out the D minor chord. We are in D minor.

The best answer to your question is to know all of the 24 keys (12 major + 12 minor) and what key signatures each has.

Like Teodor said, any and every melody (except atonal melodies... if there are such things) revolve around something called a "tone," or the home key of a piece - in c major, this key is c just as in a minor, this key is a.

As your ear improves, you'll be able to pick out melodies with resounding accuracy and if you combine that with simply noticing that the melody used all white keys but then 1 F-sharp, for instance, then you can easily recognize that the key is G major. Obviously this requires a foundational knowledge of key signatures and how to construct them.


If you'd like to learn [more] music theory (which will help you significantly on topics like these), see my thread filled with free resources here: Music Theory 101

Thanks Teodor & Bobpickle, for some very useful pointers....Just got to try it out now.

The best way of telling is at the end of the piece. How does the music close? Since closure relies on a final cadence, perfect or plagal, if you're in C it will end by moving from F to C (plagal) or G to C (perfect).

If you're in A minor it will move from D (major or minor) to A minor (plagal) or E (again major or minor) to A minor. The E chord is most likely to be a dominant seventh at a close so the G is most likely to be sharpened in a final cadence and the final chord may even use a sharpened third (a 'Picardy' third) to finish on A major instead of A minor as a major chord has greater finality.

Whatever key you write in there will a melodic pull to tonic. When you're noodling and you finish a phrase on the dominant it should set you up for a sequence descending to tonic (the melody 'wants' to end on tonic). This descent can be interrupted by moving from the supertonic down to the leading note adding to the sense of closure by delaying the final note.
In A minor the leading note will be sharpened to G# and the dominant will be E setting up a closing sequence through D, C, B, A or D, C, B, G#, A.

In C major, the leading note is B and the dominant is G setting up the descent through F, E, D, C or F, E, D, B, C.

I didn't answer the question because the OP was initially talking about noodling around at the piano and picking out melodies. You can't go by final cadences or key signatures if you are doodling by ear. I only know what I did before I had much of a handle on theory or reading. I was able to hear what the main note - the Tonic - was, and I could also hear if it was major or minor by the quality of the sound. If noodling on the piano I could find which piano key that Tonic was and then I would know, "Oh, so I've been playing that in G major / G minor." and sometimes I was right.

I was just viewing this video about hear and play techniques. It's the first of a series of four videos you get if you sign up on hearandplay.com. Not sure if it can be of help, because I myself didn't understand much of it!

https://www.youtube.com/watch?v=-zm9-YtDGqA

No, you can't go by key signatures and cadences but if you're noodling you can feel what closure is. If the music feels like it's closing from E to A then it's A minor and if it's from G to C then it's C major. I think that's what the OP was concerned about. He may not know what a cadence is but he should be able to feel when the music comes to rest.

If he finishes a phrase on E and then descends to A does it feel like the piece ends? Then try ending a phrase on G and descending to C. They'll both have a 'sense of closure' but one should feel more like the end of the song rather than just the end of a line.

I get the impression he's trying to compose without a great knowledge of harmony. I think he just wants to be able to harmonise the melodies he's coming up with. If he's uncertain of the key he's in then working his way back to tonic, even with a temporary phrase just to establish the key, is a useful tool and it doesn't need bucket loads of theory to 'get in the way' of his creativity.

I'll be the first to suggest working backwards. Pick a key. For beginner beginners, I'd suggest one of the more popular keys: C, D, G, A. Find out the notes that belong to that key. Figure out the simple triad chords and first inversions for some chords in that key. If a bit more advanced go on to the 7th chords and their inversions. Play the scales and noodle around in that key using that information.

I didn't read the responses because they look too complicated. Easy answer--99% of all pieces, especially popular tunes, end on the note that is the "key" of the piece. Also, pieces usually begin with that same note, whether its in the melody or the bass.

Ah, figured by noodling he meant he was just playing around and then discovered melodies from popular tunes or songs that he remembered. This is what I often do and then determine the key signature from the occurrence of accidentals; though, again, keep in mind these are melodies that already exist in a static form

Ok, but when I noodle around with existing melodies, I'm not necessarily noodling in the right key. If you're doing this by ear, how can you determine accidentals? You could be playing Baa Baa Black Sheep in F# (Gb) major. With the proper key signature there wouldn't be a single accidental.

I'm on a bad string of using musical terms improperly/confusingly today - I'm sorry.

I simply meant sharps/flats. The ear obviously tends to nudge you in the right direction in terms of the proper key by means of relative pitch. What I said isn't wholly invalid though. For instance, this afternoon I was working out the melody for "I'll Be Home For Christmas" and the key worked itself out to be either G or F major (which it was I can't remember), but regardless of what it was, I think I remember a G-sharp/A-flat working its way in and out of the melody which would be "hearing the accidental(s)."

Thanks Folks, for lots of ideas for me to consider.