The name Sudoku is the Japanese abbreviation of a longer phrase, "suji wa dokushin ni kagiru" meaning "the digits must remain single"; it is a trademark of puzzle publisher Nikoli Co. Ltd in Japan. Other Japanese publishers refer to the puzzle as Nanpure (Number Place), the original U. S. title.
The numerals in Sudoku puzzles are used for convenience; arithmetic relationships between numerals are absolutely irrelevant. Any set of distinct symbols will do; letters, shapes, or colours may be used without altering the rules (Penny Press' Scramblets and Knight Features Syndicate's Sudoku Word both use letters). Dell Magazines, the puzzle's originator, has been using numerals for Number Place in its magazines since they first published it in 1979. A software version called Snoodoku, by WOM Games, uses the characters from its game Snood. Numerals are used throughout this article.
The attraction of the puzzle is that the completion rules are simple, yet the line of reasoning required to reach the completion may be complex. Sudoku is recommended by some teachers as an exercise in logical reasoning. The level of difficulty of the puzzles can be selected to suit the audience. The puzzles are often available free from published sources and also may be custom-generated using software.