Futoshiki

Futoshiki (a literal translation would be 'not equal') is a Japanese puzzle that can be found in several national newspapers and made its first appearance in Saturday editions of 'The Guardian'.

Puzzles come in different sizes. In a 5x5 grid, the digits 1-5 must appear in each row and column. In a 7x7 grid, the digits 1-7 must appear in each row and column. And so on. One or two digits may be pre-placed, but solving depends on the ability to interpret the strategically placed 'greater than' and 'less than' symbols between certain cells.

The puzzle's popularity, like that of other significant Japanese puzzles, lies in its apparent simplicity. Numbers must be inserted in a grid in accordance with a few simple instructions.

Solving depends on a number of strategies that must often be used in combination. There is, for example, the basic non-duplication rule: once certain digits have been placed in a row and column, those digits can be eliminated from other cells in the row and column. It helps to note possible digits for each cell, as this is often revealing. For example, if two cells in a row can only contain the digits 2 and 4, the one of the cells must contain a 2 and the other, a 4. This means that the digits 2 and 4 can be eliminated as possibilities in other cells in the row.