A key defines the collection of pitches to be used in a given piece or section. That is, if a piece is in the key of C major, the majority of pitches will be those found in the C major scale. The tonic and dominant scale degrees are considered the most important, and since the key defines which pitches fill these roles, it also helps establish which pitches will be the most important in a given piece or section. Once we know what scale we will be using, we also know that the harmonic content will be drawn from that scale. Thus, the key indicates which pitches will constitute the majority of melodic and harmonic content in the piece.
In notation, the key is indicated by a key signature, which is a collection of sharps or flats that is placed on the staff, just after the clef. When sharps or flats are added to the key signature, they are placed on the staff at the line or space of a specific pitch. This means that every time that pitch is played, it should be raised or lowered accordingly. For example, if the key signature has a flat on the line where the pitch B is placed, it means that all the Bs in the piece become B-flats, no matter which octave they are in. In the example on the left, there are two sharps: one on the F line and the other on the C space. This means that all Fs should be played as F-sharps, and all Cs should be played as C-sharps. C major has no sharps or flats in its key signature, so every note is the default, or natural, version of itself.
It is worth noting that sharps and flats never appear together in key signatures because it would disrupt the order of whole and half steps in major and minor scales. If you would like to see the mechanics behind this system, see advanced keys and scales.
Circle of Fifths: Relationships Between Keys
This is the circle of fifths. It shows all 12 major keys and 12 minor keys possible in the Western system. The name "circle of fifths" comes from the fact that the tonic pitch of a key (which is the same as the name of the key) changes by a perfect 5th each time you add or remove an accidental: it goes up a 5th each time you add a sharp or remove a flat, and it goes down a fifth each time you add a flat or remove a sharp. That is, if we start at C, which has no sharps or flats, and go up a perfect 5th, we get G, and the key of G has one sharp. It should be noted that flats and sharps are always added in a specific order. If you would like to learn more about this, see advanced keys and scales.
Just as there are 12 discreet pitches in the Western system (and thus, in the chromatic scale), there are 12 major and 12 minor keys possible in the Western system. On this particular circle of fifths, the major keys are listed on the outer circle, and the minor keys are on the inner circle. It is important to note that when discussing keys, if major or minor is not stated explicitly, the major key is assumed. That is, when something is described simply as being in the key of C, that is understood to mean that it is in the key of C major.
The keys at the bottom of the circle of fifths have two names because they are enharmonic equivalents. The principle of enharmonic equivalence is the same for keys and scales as it was for individual pitches. Enharmonic keys occur when the same set of pitches can be indicated with either sharps or flats. For example, the key of D-flat has 5 flats and the key of C-sharp has 7 sharps. Just as the pitch D-flat is the same as C-sharp, so are the sets of pitches in their respective keys. If we look at each note in the D-flat and C-sharp major scales, we can see that each scale degree is enharmonically equivalent.
|D-flat Major Scale:||D-flat||E-flat||F||G-flat||A-flat||B-flat||C||D-flat|
|C-sharp Major Scale:||C-sharp||D-sharp||E-sharp||F-sharp||G-sharp||A-sharp||B-sharp||C-sharp|
Parallel and Relative Keys
We learned about the concept of relative and parallel relationships in the section on scales, and this is another idea that applies equally to keys. In the circle of fifths above, the keys are aligned in slices according to their key signatures. Since two keys are considered relative if they share the same collection of pitches, the major and minor keys that are aligned in each slice are relative keys (examples: G major and E minor, E-flat major and C minor).
|Key Signature:||no sharps or flats||1 sharp||2 sharps||3 sharps||4 sharps||5 sharps or 7 flats||6 sharps or 6 flats||5 flats or 7 sharps||4 flats||3 flats||2 flats||1 flat|
|Major Keys:||C||G||D||A||E||B or C-flat||F-sharp or G-flat||D-flat or C-sharp||A-flat||E-flat||B-flat||F|
|Minor Keys:||A||E||B||F-sharp||C-sharp||G-sharp or A-flat||D-sharp or E-flat||B-flat or A-sharp||F||C||G||D|
Keys in the circle of fifths that have the same tonic pitch are parallel keys. In the table below, you will see the key signatures required to have a major or minor key for each tonic pitch.
|Tonic Pitch (Key Name):||C||G||D||A||E||B or
|F-sharp or G-flat||D-flat or C-sharp||G-sharp or A-flat||D-sharp or E-flat||B-flat or A-sharp||F|
|Key Signature for Major Key:||no sharps or flats||1 sharp||2 sharps||3 sharps||4 sharps||5 sharps or 7 flats||6 sharps or 6 flats||5 flats or 7 sharps||4 flats||3 flats||2 flats||1 flat|
|Key Signature for Minor Key:||3 flats||2 flats||1 flat||no sharps or flats||1 sharp||2 sharps||3 sharps||4 sharps||5 sharps or 7 flats||6 sharps or 6 flats||5 flats or 7 sharps||4 flats|
When the key changes in a middle of a piece, this is called a modulation. Although changing from any of the 24 keys (12 major and 12 minor) to any other is a modulation, there are certain key changes that are far more common than others. The circle of fifths can help us understand which keys are closely related and therefore which are most likely to modulate to each other.
Put simply, keys that are close to each other on the circle of fifths are closely related. It is very common, both in art music and in popular music, to hear modulations between relative major and minor keys, for example. Another common modulation is moving from one key to the next going clockwise around the circle of fifths. Modulating from the key of D to the key of A would be an example of this. Counter-clockwise modulation (from the key of A to the key of D, for example) also occurs, but is less common and more surprising.
More surprising still are modulations to distant keys. Keys that are not near each other on the circle of fifths share fewer pitches and chords, which makes it difficult for composers to effect convincing and elegant modulations between them. Some composers enjoy the challenge of tackling these types of modulations. Popular music rarely modulates to distant keys, but tends to stick to standard modulations.