The rear panel usually contains 1/4 inch left and right audio outputs and one or more 5-pin MIDI inputs. Some units may have MIDI thru connections, which can be used to chain synth devices. In the 2010s and 2020s, some synth modules have one or two USB connections. Some synth modules may have left and right XLR outputs and one or more pedal inputs (for a sustain pedal and expression pedal). The front panel may also have a headphone jack, USB port, or another port for making connections. Some synth modules may have an XLR input for a vocal mic, for routing through a vocoder.