Crossword

Crosswords are a type of clue-based word puzzle, and widely considered to be one of the most popular types of word puzzle to exist. Crosswords are usually in the form of a grid of squares to be filled in with intersecting words, indicated by a series of clues separated into "Across" and "Down" entries. Over time, however, many variations that challenged that initial format have popped up, with many becoming popular for use in both newspapers and puzzle hunts.

Background
See Also: Crossword

Puzzles and games involving words crossing each other date back to at least the first century AD, with the first known appearance of a word square showing up in Pompeii prior to the eruption of Mount Vesuvius. Since then, word squares have shown up in publications for children and adults alike, and some puzzles inspired by them showed up in children's books and certain newspapers around the end of the 19th century. Crosswords as we know them today were invented in 1913 by Arthur Wynne, who called his first creation a "word-cross" (although it was soon switched to "cross-word"), and had it published in the New York World newspaper. From there, it quickly grew to be popular among readers, causing it to spread to other east-coast publications throughout the latter half of the 1910s. By 1924, the puzzle was popular enough for Simon & Schuster to publish a book of puzzles (with a complimentary pencil attached), which helped grow the desire for crosswords even more.

Similarly in 1924, crosswords reached the UK via the Sunday Express, which published an adapted version of one of Wynne's puzzles for a British audience (with the time it took being notable, as Wynne himself was from Liverpool).

(Further description of rise of the New York Times crossword (including their original stance against crosswords in the 1920s), saving any mention of Cryptic crossword history for that page)

Puzzle Application
Crossword puzzles have a wide range of "rules" for construction, the majority of which are conformed to no matter where it's being published. These include:


 * Rotational symmetry - A crossword grid should look the same when rotated 180 degrees.
 * Interconnected cells - All white squares should be connected, with no isolated sections.
 * Length - All entries should be at least 3 letters long.
 * No Duplication - A word should not show up twice in the same grid.
 * The Error That Cannot Be Named - Clues should not contain the word or phrase they're cluing.
 * Theme - Depending on the day, most crosswords will have a few "themed" entries that either follow a clue gimmick or are connected somehow.

Prevalence of black cells and word count depend on the structure being used, and clue choice can vary greatly depending on the author. However, depending on where one is submitting their crossword, there may be additional guidelines (such as the New York Times requiring a black square count of less than 17% of the grid).

Hunt crossword puzzles will often take things a step farther, and have room to play with the standard conventions, such as Crimes Against Cruciverbalism which. Theme entries are also a common place for hunts to take liberties in construction, often using them as a way to convey a message about further steps to be taken, or using them to further break what is considered "correct" in crosswords (such as including multiple letters in a single cell, having clashes at intersections, or replacing parts of words with emoji).

Structure
There are three primary schools of crossword structure, and they vary primarily based on their use of black squares as spacers and/or filler.

Blocked (American Style)
American-style blocked grids, such as the ones used for The New York Times' crossword, use as few black squares for delimitation as possible, leaving more space in the grid for words. In addition, all letters are checked, preventing a single difficult clue from stopping a full solve. The downside is that the creation of this type of crossword is much harder for the author, as they require more words to fill the space and more planning to prevent nonsense words from popping up in the fill late in the creation process.

American-style crosswords are more common in hunt puzzles, likely due to the prevalence of American puzzle hunts. However, the style does lend itself well to hunt puzzles, as the high rate of intersection (with every letter being checked by a perpendicular entry, each word is crossed the same number of times as its length) can allow for a wider range of possible extractions.

Blocked (British Style)
British-style blocked grids, such as the ones used for The Guardian's crossword, can have up to half of its squares blacked out. These grids have an almost checkerboard-like base, with the entries filling in/covering unneeded black squares to create an intersecting pattern. This style ultimately leads to only about half of entries' letters being checked on average, with it being very rare for all of an entry's letters to be checked.

British-style blocked crossword grids are commonly used for cryptics, which does tend to make solving from the checked letters somewhat easier (as wordplay does work well with partial fills). However, not all British-style crosswords are cryptics, and while this type of grid is easier to construct (fewer intersections and fewer entries means that everything can/should be relatively normal words/phrases) it can still end up being harder to solve.

British-style blocked grids are less common in hunt puzzles, but still show up from time to time, particularly as more British hunts are being created, and more British authors are writing for international hunts.

Barred
Barred crosswords have no black cells at all, instead delimiting answers using thicker lines within the grid. While this doesn't necessarily guarantee that every letter will be checked, most letters will be. In addition, every cell in the grid contains a letter, meaning that (given the same size grid), barred crosswords will always contain more text than blocked crosswords.

The lack of filler spaces in a barred crossword does make it one of the hardest types of crossword to write, as there will usually be more places to mess up the fill and create nonsense words. However, as barred crosswords do allow for unchecked letters (if a cell is bookended by two parallel bars), it does have some room for error. Overall, barred crosswords can strike a happy medium between the two blocked styles.

Similarly to British blocked grids, barred grids are often used for cryptic crosswords, overwhelmingly so in hunt puzzles. In fact, nearly every cryptic crossword that has appeared in the MIT Mystery Hunt has been either a British blocked crossword or a barred crossword, with the vast majority being the latter.

Variations
Out of all modern puzzle types, it's likely that crosswords have the most accepted variations. This is likely due to the simplicity of the original format. Most variations tend to make solving them more difficult, which makes these variations excellent for use in both puzzle hunts and regular puzzle publications, as they're a relatively simple way to play with expectations. Known variations include:


 * Arroword - Instead of numbering starting squares for clue assignment, arrows are placed in the grid along with the clues themselves (confined to single squares), often resulting in less page space taken up by the clues, but smaller or easier puzzles.
 * Codeword - No clues, and around half the letter going unchecked. Instead, each square contains a number, and each number represents the same letter everywhere it appears in the grid.
 * Criss-Cross - Eschews both the convention of having a symmetrical grid and the convention of having all spaces be cross-checked by another entry. Criss-crosses instead contain entries that will usually cross much fewer other entries than it has space for, creating a very spread-out puzzle with a heavier reliance on pure knowledge (since most entries will have only a few checked letters to work off of).
 * Cryptic Crossword - Clues read as nonsense at face value, but can be split into a definition half and a wordplay half, with both solving to the same answer. Often barred rather than blocked.
 * Diagramless Crossword - Clues are as normal, including numbering, but a grid is not provided and must be created based on the logical placement of the different entries.
 * Fill-In - Instead of clues, entries are explicitly given. Entries are not numbered, so they must be logically placed within the grid.
 * Marching Band - Square grid, divided into concentric square bands. Clues are divided into across (which go across the rows, and may contain multiple clues to fill the space) and "marching bands" which travel clockwise around each of the bands.
 * Rows Garden - Grid is divided into triangles, and form a series of hexagonal "flowers". Clues are divided into across (which go across rows) and "buds" or "flowers", which travel a particular direction around the centers of the hexagonal flowers.
 * Siamese Twins - Two identical crossword grids with one set of clues. Each number contains two distinct clues of identical answer length, forcing solvers to separate the answers into two sets that form different fills for the grids.
 * Some Assembly Required (SAR) - Puzzle containing no black squares, and only across clues for the completed grid. The grid get broken up into 1-block-width fragments, jigsaw puzzle-style, where each fragment contains a word reading from one end to the other. These fragments are provided, numbered, and clued, allowing cross-checking of every cell in the final puzzle.
 * Something Different - Otherwise a normal crossword, except with clues that do not solve to dictionary-definition answers. Instead, the clues may be puns or nonexistent phrases, and solvers musty rely on intuition and heavy cross-checking to solve.