On a photocopier or computer printer, duplex refers to the ability to print on both sides of each piece of paper. Most often, this is done by printing one side, shunting the paper into a special tray or duplexer unit, shunting it back in such a way that the other side is up, then printing the other side.

Because the addition of a duplexer increases the complexity of the printer's paper path, it slightly increases the chance of a paper jam. However, printing in duplex cuts in half the amount of paper you need to print your documents. Save a tree -- print in duplex.

A duplex can also be a house made of exactly two apartments. The apartments in a duplex are frequently substantially identical; sometimes they are mirror images of one another.