PBT (short for "polybutylene terephthalate") is one of the hardest, most durable materials for keycaps. It is more resilient against heat and chemicals than ABS, but its properties also makes it more difficult to mould, which makes it more uncommon.
Legends on PBT keys are often laser-etched or dye-sublimated but only very rarely double shot moulded. PBT does not turn yellow from exposure to ultraviolet light, like ABS does. The resilience against heat makes it a good candidate for dyeing in a hot dye bath.
Double-shot moulding, double-shot, dual-injection moulding, two-shot moulding or insert moulding is the process of moulding plastic around a preformed metal or plastic insert. This has been a popular process to create very durable keycaps in the past. Recently, the process has become more rare.