A crossword (or crossword puzzle) is a word game consisting of a grid of black and white squares, into which solvers enter words or phrases ("entries") crossing each other horizontally ("across") and vertically ("down") according to a set of clues. Each white square is typically filled with one letter, while the black squares are used to separate entries. The first white square in each entry is typically numbered to correspond to its clue.
Crosswords commonly appear in newspapers and magazines. The earliest crosswords that resemble their modern form were popularized by the New York World in the 1910s. Many variants of crosswords are popular around the world, including cryptic crosswords and many language-specific variants.
American-style crosswords
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 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).
Substantial variants from the usual forms exist. Two of the common ones are barred crosswords, which use bold lines between squares (instead of shaded squares) to separate answers, and circular designs, with answers entered either radially or in concentric circles. "Free form" crosswords ("criss-cross" puzzles), which have simple, asymmetric designs, are often seen on school worksheets, children's menus, and other entertainment for children. Grids forming shapes other than squares are also occasionally used.
Puzzles are often one of several standard sizes. For example, many weekday newspaper puzzles (such as the American New York Times crossword puzzle) are 15×15 squares, while weekend puzzles may be 21×21, 23×23, or 25×25. The New York Times puzzles also set a common pattern for American crosswords by increasing in difficulty throughout the week: their Monday puzzles are the easiest and the puzzles get harder each day until Saturday. Their larger Sunday puzzle is about the same level of difficulty as a weekday-size Thursday puzzle. This has led U.S. solvers to use the day of the week as a shorthand when describing how hard a puzzle is: e.g. an easy puzzle may be referred to as a "Monday" or a "Tuesday", a medium-difficulty puzzle as a "Wednesday", and a truly difficult puzzle as a "Saturday".
Typically clues appear outside the grid, divided into an across list and a down list; the first cell of each entry contains a number referenced by the clue lists. For example, the answer to a clue labeled "17 Down" is entered with the first letter in the cell numbered "17", proceeding down from there. Numbers are almost never repeated; numbered cells are numbered consecutively, usually from left to right across each row, starting with the top row and proceeding downward. Some Japanese crosswords are numbered from top to bottom down each column, starting with the leftmost column and proceeding right.
Clues: conventions and types
American-style crossword clues, called straight or quick clues by those more familiar with cryptic puzzles, are often simple definitions of the answers. Often, a straight clue is not in itself sufficient to distinguish between several possible answers, either because multiple synonymous answers may fit or because the clue itself is a homonym (e.g., "Lead" as in to be ahead in a contest or "Lead" as in the element), so the solver must make use of checks to establish the correct answer with certainty. For example, the answer to the clue "PC key" for a three-letter answer could be ESC, ALT, TAB, DEL, or INS, so until a check is filled in, giving at least one of the letters, the correct answer cannot be determined.
In most American-style crosswords, the majority of the clues in the puzzle are straight clues, with the remainder being one of the other types described below.