On manual transmissions, the clutch is a mechanical device–operated by the clutch pedal–that engages and disengages the engine from the transmission so that you can change gears at idle without moving, etc. It generally consists of a clutch disc, a pressure plate, and a flywheel.
Automatic transmissions also have clutches, but in clutch packs. These are very small compared to a manual transmission. Each gear has it’s own clutch pack and the valvebody controls which one is activated or not.
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