 A deck decompression chamber (DDC), or double-lock chamber is a two compartment pressure vessel for human occupation which has sufficient space in the main chamber for two or more occupants, and a forechamber which can allow a person to be pressurised or decompressed while the main chamber remains under constant pressure.This allows an attendant to be locked in or out during treatment of the occupant(s) of the main chamber.There is usually also a medical lock, which serves a similar function but is much smaller.