The basis of traditional forms of improvisation is to create spontaneously and play melodies that are built on the basic chord progression of the song. At the most basic levels, the notes you choose for your improvisation are partially dictated by the scale associated with each chord. This is called playing changes. More advanced forms of improvisation give the performer more melodic and harmonic freedom, either by reducing the number of chord changes, or by making the chords progressions more ambiguous in tonality, to the point of eliminating these structures entirely. These approaches are discussed separately below.
Pianists, guitarists, or other instrumentalists who accompany themselves while improvising should read the section on accompanying along with this section and try to apply both sets of concepts at once when improvising.