fill-in

Starting grid for a simple Fill-In. The word list is:
GI IO ON OR
DAG EVO OED REF
ARID CLEF CLOD DAIS DENS DOLE EDIT SILO
ARTICLE VESICLE

Fill-Ins (also known as Fill-It-Ins) are a type of logic puzzle commonly printed in various puzzle magazines. While they superficially resemble crossword puzzles, they do not require any form of "outside knowledge"; unlike a crossword, one can solve a Fill-In in a non-native tongue, even one with an alternate character set.

Rules

Fill-Ins are presented in two parts: the grid, which typically resembles an empty American crossword (with one word possibly already entered as a starting point) and the word list. The word list is traditionally sorted first by length and then alphabetically, so as to make finding a particular word as simple as possible.

The goal is to "fill in" the grid with the words in the word list. Words are entered horizontally and vertically, as in crosswords, but the word list gives no indication of location or direction; the puzzle aspect comes from determining the placement of all the words.

Solution methods

Solved grid for the above Fill-In.

Solving a Fill-In typically amounts to searching for words of a certain length with letters in specific places. If a starter word is given in the grid, it is often useful to use it as the beginning search point, but long words are more generally useful (especially if there's only a few of them in the word list). There are usually many three- and four-letter words in a puzzle; using longer words first often makes the placement of shorter words easier.

Raw "trial and error" is best used when there are only two or three words that can potentially fit at a given location; temporarily assume one of the words, and see if an impossible letter combination results. If so, that word is not the one that should go in the grid at that location. Unlike Sudoku, most moderately difficult Fill-Ins require a number of these "trial and error" explorations to solve.

Care must be given to marking out words that are not explicitly placed in the grid; this occurs when one fills in a vertical sequence of horizontal words, or vice versa. Forgetting to do this results in "extra words" and often makes the puzzle more difficult to solve.

Variations

A common variation on the standard Fill-In is using numbers, or a mix of numbers and letters, instead of specific words. As the words in the puzzle do not "mean anything," such puzzles are effectively identical to the standard type of puzzles, with more or less symbols.

Particularly large Fill-Ins may list 'Across' and 'Down' words separately, to make the search for matching words require less "trial and error."

A picture of the inside of a Fill-In.

Also common is the replacement of a standard American crossword grid with a more open, "loose crossword" format (as is common with crosswords used in classrooms or generated by computer programs), where there is no symmetry and every symbol is not necessarily covered in both directions. Often the black squares are omitted in this form (unless needed to disambiguate a close cluster of squares); these are commonly marketed under the names Frameworks or Kriss-Kross.

A more complex variation on the Fill-In is the Diagramless Fill-In, where the grid is initially empty except for all instances of a single letter (typically a vowel). Like diagramless crosswords, these puzzles rely on the rotational symmetry of American crossword grids to help the solver determine the location of the various words.