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What does Su Doku actually mean? None of my Japanese colleagues seem to know the original Japanese name. Ryoko Attwell, Buckinghamshire

You won’t find Su Doku in a Japanese dictionary. It’s a coined word, made up by a Japanese publisher – just like advertising men here make up words like KwikRite and StopzIt. I don’t speak Japanese, but as I understand it Su stands for number, and Doku has some notion of singularity or bachelorhood about it, emphasizing perhaps that there is only one of each number in each unit (row/column/box).

Can there only ever be one solution? Andrew Marjoribanks, Greenock

Yes. In a properly-formed Su Doku puzzle, there is only one solution. All Su Doku puzzles supplied to The Times are properly formed. However, puzzles from other sources could be faulty.

It is now quite clear that several of the published puzzles have multiple solutions. Originally you promised that each puzzle would have a unique solution: I wonder whether you could comment? Katherine Edgcombe, Cambridge

Yes, I’d be glad to. It is not correct that puzzles published in The Times have multiple solutions. If it seems that they do, I guess the puzzles are doing their job of puzzling you. From time to time people claim to have found a puzzle with multiple solutions. I have checked all cases that have been sent to me, and they are all unfounded. Here are areas where people go wrong:

1. Not understanding the rules. This was a problem in the first few weeks, but not such a problem now. Some people did not appreciate that boxes must comply with the “1 through 9” rule, not just rows and columns.

2. Letting clues move. Clues are fixed. If your alternative solution is created by swapping pairs of numbers, check to make sure that one of the numbers is not a clue.

3. Transcribing the puzzle wrongly. For various reasons people apparently make copies of the puzzle and solve the puzzle on the copy. But in making the copy, they miss out a clue, or put it in the wrong place, or enter the wrong number. This means that the puzzle they solve is not the puzzle which was set. It’s not surprising that the copy-puzzle won’t come out, or that it will but it has multiple solutions.

A quiet word to those who think they have found a puzzle with multiple solutions: it might be best to email me first before going public. I can tell you what you're missing.

How are the puzzles' difficulty graded? Is it a rigid mathematical reason or just based on opinion? Name and address withheld

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