As an improviser, you must know which scales to use over a given chord when soloing. For example, if you are playing over a G7 chord, the mixolydian mode would be the best scale to use against this chord. A G7 chord is constructed using a G (root), B (3rd), D (5th) and F (b7). You can also add optional extensions A (9th), C (11th) and E (13th) to give the G7 chord different colors within the key. The G mixolydian scale is G,A,B,C,D,E,F, so all the chord tones and extensions are contained within.