A washer-dryer is one single machine that both washes your clothes and then dries them, so you can put wet things in and take dry things out without moving them. It is popular where there is no space for two separate machines — small flats, city apartments, Britain generally, where it usually sits under the kitchen counter. The honest truth, which the shop will not tell you, is that the drying half is the weak half: these machines dry a smaller load than they wash, take a long time, and often leave clothes a bit damp, so people who have room usually prefer a separate washing machine and a separate dryer. Americans more often say "washer-dryer" about the pair of machines sold together, which is a real source of confusion — if you must be clear about the single machine, say "a combined washer-dryer" or "a combo unit". Also written "washer dryer" and "washer/dryer".