Rebreather divers and some open-circuit scuba divers carry extra diving cylinders for bailout in case the main breathing gas supply is used up or malfunctions. If the bailout cylinder is small, they may be called "pony cylinders". They have their own demand regulators and mouthpieces, and are technically distinct extra scuba sets. In technical diving, the diver may carry different equipment for different phases of the dive. Some breathing gas mixes, such as trimix, may only be used at depth, and others, such as pure oxygen, may only be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported by a backplate while others are side slung from strong points on the harness.