A crossword is a word puzzle that normally takes the form of a square or rectangular grid of black and white squares. The goal is to fill the white squares with letters, forming words or phrases, by solving clues which lead to the answers. In languages which are written left-to-right, the answer words and phrases are placed in the grid from left to right and from top to bottom. The black squares are used to separate the words or phrases.

Terminology

The horizontal and vertical lines of white cells into which answers are written are commonly called entries or answers. The clues are usually called just that, or sometimes definitions. White cells are sometimes called lights, and the black cells are sometimes called darks, blanks, or blocks.

A white cell that is part of two entries (both Across and Down) is called checked, keyed or crossed. A white cell that is part of only one entry is called unchecked, unkeyed or uncrossed.

The creating of crosswords is called cruciverbalism among its practitioners, who are referred to as cruciverbalists, from the Latin for cross and word. Although the terms have existed since the mid 1970s, non-cruciverbalists rarely use them, calling crossword creators constructors or (especially outside the United States) setters or compilers.

Types of grid

Image:CrosswordUSA.svg|American-style grid Image:CrosswordUK.svg|British-style grid Image:CrosswordJPN.svg|Japanese-style grid Image:Schwedenrätsel.jpg|Swedish-style grid

Crossword grids such as those appearing in most North American newspapers and magazines feature solid areas of white squares. Every letter is checked, and usually each answer is required to contain at least three letters. In such puzzles black squares are traditionally limited to about one-sixth of the design. Crossword grids elsewhere, such as in Britain and Australia, have a lattice-like structure, with a higher percentage of black squares, leaving up to half the letters in an answer unchecked. For example, if the top row has an answer running all the way across, there will be no across answers in the second row.

Another tradition in puzzle design (in North America and Britain particularly) is that the grid should have 180-degree rotational symmetry, so that its pattern appears the same if the paper is turned upside down. Most puzzle designs also require that all white cells be orthogonally contiguous (that is, connected in one mass through shared sides, to form a single polyomino).

The design of Japanese crossword grids often follows two additional rules: that black cells may not share a side and that the corner squares must be white.