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As we saw on the Scales, Modes, and Chords page, there are several related chords in each key. Indeed, in the earliest harmonized music, it was common for songs to consist entirely of chords from a single key center.
For example, if a song is in the key of C, then it is common for the harmonies to include C and F major, D, E, and A minor, and G dominant. If a song uses only these chords, then an improvised melody can be built entirely from the notes of the C major scale because all of those chords fall within the C key center.
In more modern music, and especially in jazz, it is common for songs to migrate from one key center to another, sometimes moving to a new key center in every measure. To successfully improvise with these more complex song forms, one must realize how the key center is changing throughout the song. The Key / Chord Relationships chart is a tool to help you recognize the changing key centers. In jazz music, generally only one key signature is written at the beginning of the song. That is likely to be the key center at the beginning and end of the song, but the key center may change many times within the song. When an improvised solo is called for, the composer will usually write the underlying chords. This will help you find the changing key center.
For example, if you see this chord progression:
| Dmin7 | G7 | Ebmin7 | Ab7 | Dmin7 | G7 | CMaj7 |
The song would seem to be in the key of C, however measures 3 and 4 shift to the Db key center. In measures 1, 2, 5, 6, 7 use the notes from the C major scale. In measures 3 and 4, the notes from the Db major scale fit better.
Please note that there can be ambiguities. For example, the Dmin7 chord could be the ii chord in the key of C or the vi chord in the key of F. In such cases, look at the adjacent chords to help judge which key center dominates.
