In a crossword, the more words you fill in, the more help you have for answering the remaining words. This is possible because of the grid's property of "checking", i.e. the interlocking of letters between the ACROSS and DOWN clues.
Take for example, the adjacent image of the top-left corner of a typical grid. Suppose you begin at 1A and cannot answer the clue: It might bring colour to one's face (8). You move on to attempt the crossing Downs. Each intersecting clue answered in the Down direction contributes a letter for 1A. With three Down clues answered, you have a much easier task, of finding a word that looks like L?P?T???. You also have the pattern N?E? for 12A in place, though you have not yet seen the clue.
The odd letters of 1A/12A are called checked letters (i.e. letters shared with words in the opposite direction). The even letters of 1A/12A are unchecked letters or unches (i.e. letters not shared with any other word).
Rules For Checking
The amount of checking in the crossword grid influences the solvability of the puzzle. Quick crossword grids have 100% checking, so some answers reveal themselves even before their clues have been attempted by the solver. Cryptic crossword grids have a different set of principles for checking. There are minor variations between publications but on the whole the rules are fairly universal.
- No more than two unchecked letters in a row
- Roughly half the letters checked in every word. In an 8-letter word, at least 4 letters will be checked. Where the word length is odd, some publications might round down the number of checking letters to just below half; those with stricter standards of fairness like The Times and Gridman's grids in The Hindu round it up. This means that in a 7-letter word, at least surely 3 will be checked.
- If two unches occur together, the Times grid has an additional rule that they will not be the first two or last two letters of the word.
Long DOWNs, Anyone?
Chances are, you find yourself getting stuck at 8D/10D with puzzles based on the grid. Remember SKETCHY MEAL? GREEN KEEPER?
This grid has three unchecked letters in a row, breaking the first rule of fair checking. I know of no other daily crossword that would consider this grid publishable.