Where Sudoku came from
The puzzle began as Number Place in an American magazine in 1979, got the name Sudoku in Japan in the 1980s, and went around the world from 2004, when newspapers began printing it daily.
Scan for singles first
Start without notes. Take one digit at a time and see where the copies already on the board leave room for another. A digit with one possible cell in a box, row or column is a hidden single. A cell with one possible digit is a naked single. Easy puzzles need nothing else.
How to start a puzzle Hidden singles Naked singles
“Before anything clever, finish the obvious. Most grids give up more than you expect.”
Orion
Write notes and look for pairs
When the singles run out, write each cell’s candidates as notes and look for small groups. Two cells that share the same two candidates, or two digits that share the same two cells, let you clear notes around them.
Pointing and box-line reduction
Where a box and a line overlap, a digit confined to the overlap can be cleared from the rest of the line or the rest of the box.
This is the step solvers miss most often. When a medium puzzle stalls, check each box for it.
The X-Wing
On hard grids, one digit can be tied to the four corners of a rectangle, across two rows and two columns. Finding the rectangle clears that digit from the rest of both columns.
Solving habits that save time
- Go back to scanning after every hard deduction; one elimination often opens several singles.
- Keep notes current. Clear candidates as soon as you place a digit.
- If the board reports a conflict, undo back to your last certain move rather than patching around it.
