Across and down
Across and down are the two directions in which a crossword answer can run — across means left to right along a row, and down means top to bottom along a column.
Numbers in the grid mark where entries begin. A square gets a number if it starts an across entry, a down entry, or both, and the numbering runs left to right along each row and then down the grid, so the numbers are shared between the two clue lists rather than separate. That is why 1-Across and 1-Down usually begin in the same square, and why a grid can jump from 12 to 14 in one list — 13 started an entry in the other direction only.
Every square that belongs to both an across answer and a down answer is called a crossing, and its letter has to satisfy both at once. That single property is what makes a crossword self-checking: a wrong answer stops fitting the moment a crossing answer arrives, and a half-solved grid gives you letters for free.
British and American grids differ in how heavily they cross. American grids interlock almost every square; British grids leave many squares in one direction only, which is why a British solver gets less help from crossings and more from the clue.
Daily Mini Crossword — also published in a large print edition.