• The word key in music has two separate meanings; the first meaning is concrete and easily understandable, the second is abstract and difficult to explain:
  1. The lever that is pressed by the fingers to make a sound on a keyboard instrument is called a key.
  2. A set of note pitches which are used by a piece of music and which form a scale defines a keynote and therefore the key of the piece of music.

• In the second meaning above, the word key is used in the same sense as it is used in the English words keystone and keynote:
  • A keystone, being the middle stone of an arch, figuratively means a central principle
  • The phrase keynote speech seems to have become popular in recent years, meaning a speech that spells out the main or dominant principle of a group or party
  • The key of a piece of music is something on which the piece is based.
  • When a piece of music is heard, the keynote is felt to be the "home" frequency or pitch, that is the pitch of the note on which the piece should finish.

• The key of a piece of music is given by its key signature
  • This says what accidentals are in the piece by default

• A piece of music will nearly always contain the keynote in its first and last chords

• There are at least two, and sometimes three, hierarchical levels of key in a piece of music of any length or complexity:
  1. The key of the piece, sometimes quoted as part of the title
     • For example, "Sonata in C major"
  2. The key of a movement or section
     • A sonata in C major may have a section in A minor
  3. The key of the music at any point in time
     • A few bars in the A minor section might be in E major

How do you know what the key of a piece of music is?

When an experienced musician looks at a piece of "traditional" music, he/she can immediately recognise the key of the piece, and bring to mind the "feel" of that key, i.e. the accidentals in the scale and the feel of the scale under the fingers (depending on the instrument he/she plays). This is really no different from a non-musician looking at the word "blue" and bringing to mind the colour that it represents, and the emotions that it envokes.

So how is it done? Different people will have slightly different techniques, but it will generally work like this:

1. The key signature. The are only 15 different key signatures, and, of these, several are very unusual, and a few are very common indeed, so a musician will know all the possible key signatures and what they mean. However, this does not give the definite answer to "what is the key of the piece"; more information is needed.
2. The start and end of the piece. As a general rule, a piece will always start and end in the key of the piece. If the music is chordal or homophonic, it should be easy to look at the notes in the first chord and the last chord. In particular, the bass note of the last chord of the piece will always be the root note of the key of the piece.
3. A more general view of the notes that make up the scale of the piece. This is more useful when the piece is polyphonic, or not chordal, perhaps a single melody line. You cannot then rely on the note at the start or end of the piece, you must look in more general terms at the scale used at the beginning and the end. Bear in mind that a piece can modulate, that it, change its key in the middle. So for example, if you see a piece of music which consists of just a tune, and it has a key signature of one sharp, then the key could be G major or E minor. Say the first note is B and the last note is G; both of these appear in the triads of both keys, so that does not help. So look at the first few and last few notes. Are the Ds sharp or not? If they are, the key is E minor. If they are not, the key is G major.