An arpeggio is a chord unfolded in time. Instead of sounding all notes at once, the performer plays them in sequence — from bottom to top or top to bottom.
In piano music, arpeggio is the backbone of accompaniment: Beethoven's famous Moonlight Sonata is built almost entirely on arpeggios. On guitar the device is called fingerpicking, and it is one of the first technical skills in the classical guitar school.
The word comes from the Italian arpeggiare — “to play the harp,” for on the harp a chord is almost always spread out. In notation, an arpeggio is marked with a wavy vertical line before the chord. Unfolding a chord lets the ear catch each pitch separately while still sensing the harmony as a whole, which makes the arpeggio an indispensable basis of accompaniment from the Baroque to today's pop.
