A two-piece is clothing made of two matching parts that are worn together and sold together. For a man's suit, that means a jacket and trousers cut from the same cloth — a two-piece suit — as opposed to a three-piece, which adds a waistcoat. Shops and tailors use the word constantly, and in a shop you can drop the noun and just say 'a two-piece' and everyone knows you mean the suit. The same word covers a woman's matching skirt-and-jacket, and — in everyday speech, especially in shops and adverts — a two-piece swimsuit, which is simply a bikini said politely. Outside clothes, 'two-piece' can describe anything that comes in two parts: a two-piece drum kit, a two-piece bathroom set. It goes before the noun, and the noun after it stays singular: a two-piece suit, not a two-pieces suit.