The portion of the envelope of a played note following the attack and decay (or any other combination of initial transients) in which the volume level remains constant. With most ADSR envelope generators, the sustain phase lasts an indefinite amount of time; it begins after the end of the decay, and persists as long as the corresonding key on the keyboard is held down. Some synths, notably many Yamaha models, have a provision that allows the volume level to be gradually be ramped down during the sustain period; the purpose of this is to simulate the note envelopes of accoustic instruments that are not capable of infinite sustain, such as piano or percussion.