Sudoku Solving Techniques, Tips and Tricks
Nine techniques, listed in the order you actually need them. Each one solves puzzles the previous one cannot, and each is worth getting comfortable with before moving on — an expert puzzle is not a wall of exotic patterns, it is mostly the basics with two or three harder moments buried in it.
The techniques, in order
Cross-hatching
Sweep one digit across the grid and let the rows and columns you already have rule out squares for you.
Needed from Easy onwards
Naked single
One square, one surviving candidate. The most direct placement in Sudoku.
Needed from Easy onwards
Hidden single
The only square in a row, column or box that can hold a digit — even when it still shows other notes.
Needed from Medium onwards
Naked pairs
Two squares, the same two candidates: those digits belong to them, so erase them everywhere else in the unit.
Needed from Hard onwards
Hidden pairs
Two digits that can only go in the same two squares — so everything else in those squares goes.
Needed from Hard onwards
Pointing pairs
A digit cornered onto one line inside a box clears that line in the two neighbouring boxes.
Needed from Hard onwards
Box/line reduction
The same overlap argument run the other way — from the line into the box.
Needed from Hard onwards
X-Wing
Four squares in a rectangle that force a digit into two columns, clearing both of them elsewhere.
Needed from Expert onwards
Colouring and chains
Follow true/false links across the grid until a contradiction settles the digit.
Needed from Expert onwards
How the order works
The first three techniques place digits. The next four place nothing at all — they only erase candidates, which is what lets the placing techniques bite again. That is the single most useful thing to understand about the middle of this list: when a hard puzzle stalls, the move you need is usually an elimination, not a placement.
The split by difficulty is roughly this. An easy puzzle falls to scanning alone. A medium one is built so that scanning runs out and hidden singles reopen it. Hard puzzles need the subset and intersection techniques. Only expert grids reach for the X-Wing.
What about guessing?
You never need it. Every puzzle generated here is verified to have exactly one solution reachable by deduction. If you find yourself wanting to guess, there is a deduction available somewhere that you have not spotted — most often a hidden single, or a stale pencil mark you forgot to erase after an earlier placement.
Stuck for real? Take the hint. Three per puzzle, 75 points each, and no shame in it. Then go back and work out why that digit belonged there — that is where the improvement actually happens.
New to all this? Start with the rules, then come back. Ready to use them? Open a board, or take a printable sheet away with you. To check a grid from somewhere else, the Sudoku solver will say whether it has one solution at all.