A fill-in is a musical filling and occurs mostly in areas where there is a small "gab" in the music, where the melody keeps a little break and only the chords are heard. Certain fill-in patterns has the nature of beeing special cliches in various styles and tempos, and with the books Chords On Piano I've tried to categorize some typical types of fill-ins. This categorization has the word "principle" attached to it, to emphasize that the fill-ins as described, could easily be somewhat different, and that you should even try to develop your own set of fill-in figures based on the these principles.

The good news is that many of these fill-ins are relatively easy to play, and at the same time have a great effect on the musical accompaniment. Additionally, the use of fill-ins helps to develop the player's improvisational abilities, because one should not play certain figures in certain places, but develop a repertoire of fill-in figures which improvisational can be "sprinkled" over the gabs of the song.

The fill-in principles are categorized according to severity, thus occurs the arpeggio principle to be the easiest one.