Word puzzles come in many forms, but one of the most common is the crossword. A crossword puzzle is made up of a diagram that is often rectangular and is split into squares that are either blank (white) or cancelled (black, coloured, or crosshatched). This diagram is accompanied by two numbered lists of definitions or clues; one list is for the horizontal words, while the other list is for the vertical words. The numbers on these lists match the same numbers on the diagram. It is necessary to place a certain letter of the alphabet into each empty square of the picture to produce words corresponding to the numbered definitions or clues. The puzzle gets its name from how the words cross or interlock.