Improvising is one of the great creative outlets available to musicians. Often a musician will improvise a solo while another plays chords. The soloist chooses notes to play over the chords based on intuition, experience, and a few simple rules.
Play Arpeggios Over Chords
A simple way to improvise is to play individual notes of a chord while the chord is sounding during a song. For instance, when a measure has Cmaj7 in it, you could play a riff of individual notes made up of C, E, G, B. As long as you stick to these notes while the chord is playing, your improvising will harmonize with the song.
Play Scales and Modes Over Chords
Another way to improvise is to play individual notes of the scale or mode on which the chord is based. For instance, since Dm7 is based on the Dorian mode, you could play riffs using any notes from that mode.
Play Over Dominant and Altered Chords
Sometimes it is appropriate to play scales or modes that are slightly different from the scale or mode the chord is based on. By doing this, you introduce notes that alter the harmony, usually making it more dissonant. Because altered chords usually sound dissonant, they often make good dominant chords.
The example below shows the notes of G mixolydian against the Gdom7 chord. Just below that is the G phrygian natural-3 mode (from the harmonic minor scale). Playing this mode over a Gdom7 will make it sound like a Gdom7b9 by adding the flatted 9 to the harmony.
Below is a table of chords with suggested scales and modes appearing on the Harmonizer that can be played over them. The root of the chord becomes the beginning note of the scale or mode. Ex: Play G Mixolydian over a Gdom7 (G7) chord.