A combinatorial puzzle related to Latin squares.
A Latin square is an $n$-by-$n$ array filled with $n$ different symbols, each occurring exactly once in each row and exactly once in each column.
Sudoku and KenKen puzzles form special cases of Latin squares; any solution to a Sudoku or a KenKen puzzle is a Latin square. (However, it is unnecessary to use this tag with every question relating to puzzles of these types.)
It is NP-complete (i.e. computationally intractable) to decide whether a partially filled square can be completed to a full Latin square.
The origin of the use of 'Latin' in the name dates back to the mathematician Leonhard Euler, who used Latin characters as symbols.
