A duplex nail is a two-headed nail that was invented in 1916 by William Arthur Collings to replace common nails used in formwork. Before the invention of duplex nails, common nails were widely used in formwork. Today, duplex nails are most commonly used in temporary construction, such as building wooden scaffolding, formwork, or temporary wood braces. The nail is hammered only to the first head leaving a gap with the second so the nail can be easily removed using a hammer.