A feature found in some keyboards, or a stand alone device, that automatically plays musical parts. Originally this was in the form of static drum beats played by an organs. Later it involved more instruments (bass, strings, etc.), but these days the feature “auto accompaniment” is understood to mean a full featured musical performance of many instruments that’s dynamic. In other words, the musical performance can react to realtime input from the user. For example, playing a certain note on a keyboard will cause the accompaniment to base its musical parts around that note as if it’s the key of the song. Some can react to chords or even a series of chord changes, and play relevant bass lines, string and/or horn section material and more. Some of the more sophisticated designs actually border on artificial intelligence in that they can continually make up completely new arrangements based on little to no user input, or a great deal of user input. In other words, it’s almost like having a real band of great musicians constantly coming up with ideas on the fly. A surprising number of hit songs are actually “co-written” by the auto accompaniment algorithms built in to some keyboards.