Crosswords

A crossword is a word puzzle that normally takes the form of a square or a rectangular grid of white and black shaded 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 that 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 shaded squares are used to separate the words or phrases.

American-style grid

British/South African-style grid

Japanese-style grid

Swedish-style grid

Barred grid where bold bars are used instead of shaded blocks to separate the words

Crossword grids such as those appearing in most North American newspapers and magazines feature solid areas of white squares. Every letter is checked (i.e. is part of both an "across" word and a "down" word) and usually each answer must contain at least three letters. In such puzzles shaded squares are typically limited to about one-sixth of the total. Crossword grids elsewhere, such as in Britain, South Africa, India and Australia, have a lattice-like structure, with a higher percentage of shaded squares (around 25%), leaving about half the letters in an answer unchecked. For example, if the top row has an answer running all the way across, there will often be no across answers in the second row.

Another tradition in puzzle design (in North America, India, and Britain particularly) is that the grid should have 180-degree rotational (also known as "radial") 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 shaded cells may not share a side (i.e. they may not be orthogonally contiguous) and that the corner squares must be white.

The "Swedish-style" grid (picture crosswords) uses no clue numbers, as the clues are contained in the cells which do not contain answers. Arrows indicate in which direction the clues have to be answered: vertical or horizontal. This style of grid is also used in several countries other than Sweden, often in magazines, but also in daily newspapers. The grid often has one or more photos replacing a block of squares as a clue to one or several answers, for example, the name of a pop star, or some kind of rhyme or phrase that can be associated with the photo. These puzzles usually have no symmetry in the grid but instead often have a common theme (literature, music, nature, geography, events of a special year, etc.)

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Wikipedia