A style of envelope generator used particularly by EMS. The envelope it generates has four segments. The attack segment rises from zero level to maximum level in an amount of time set by the attack knob. The "on" segment remains at the maximum level for a fixed amount of time determined by the on knob; in the canonical EMS trapezoid, the on segment is not a sustain segment as it does not hold indefinitely waiting for a gate signal to drop. The decay segment drops from the maximum level to zero level in the amount of time set by the sustain knob. There is a fourth knob marked "off"; if this is at its maximum setting, the output remains at zero level until the trapezoid is triggered again. However, if the knob is not at its maximum, it determines an amount of time to remain off, after which the trapezoid will re-trigger itself and began with the attack segment again. In this mode, the trapezoid acts as an LFO generating a wave shape determined by the four parameters.