A crossword puzzle consists of a diagram, usually rectangular, divided into blank (white) and cancelled (black, shaded, or crosshatched) squares. This diagram is accompanied by two lists of numbered definitions or clues, one for the horizontal and the other for the vertical words, the numbers corresponding to identical numbers on the diagram. Into each of the blank squares of the diagram a certain letter of the alphabet is to be inserted, forming the words fitting the numbered definitions or clues.