The difference between scales and keys

Both scales and keys refer to groups of tones and sometimes we can either say scale or key and mean the same thing. Still, there is a difference.

A key is a collection of tones and it is common to say that a piece of music is written in the key of… For example, a song that includes the chords G, C, D and Em are most typically in the key of G. That is because all the tones that belongs to the four chords are also belonging to the key of G (which are G, A, B, C, D, E, F#).

It is true that also the G Major scale includes the same tones: G, A, B, C, D, E, F#. But on the same time could a song in the key of G include several scales. For example, in a blues song in the key of A may the guitarist use both the pentatonic major and minor scale. The guitarist could also use the pentatonic scale in A, D and E. So, in a song that goes in a specific key more than one scale could be used.

Another thing that separates keys and scales are that we refer to quite a few keys, which tends to be in major or minor. But with scales, where is a large diversity. Yes, we talk about major and minor scales, but also about diatonic scales, mixolydian scales, bebop scales and so on. A pianist may choose to use the Eb Bebop scale in a jazz tune, but we don't say about a jazz tune that it was in the key of Eb Bebop.

Music scales could be seen as tools for creating music. For example, to improvise with and the groups of tones in a scale are in this case setting up directions for the musician. Music keys, on the other hand, is more a way to describe music.