The linotype machine operator enters text on a 90-character keyboard. The machine assembles "matrices", which are molds for the letter forms, in a line. The assembled line is then cast as a single piece, called a "slug", from molten type metal in a process known as hot metal typesetting. The matrices are then returned to the type magazine from which they came, to be reused later. This allows much faster typesetting and composition than original hand composition in which operators place down one pre-cast glyph (metal letter, punctuation mark or space) at a time.