Sometimes a washing machine and a clothes dryer are put together. The washing machine is usually on the bottom of the stack because the water in it makes the machine heavier than the dryer. In this arrangement the washing machine may be front-loading or top-loading. For the top-loading washing machine, room must be left to open the door at the top to add the clothing to be washed. A front-loading washing machine can have the dryer sitting on top of it.