How to Play Sudoku: Three Rules and Your First Proved Move

A small set of rules, repeated across the grid

A classic Sudoku has 81 cells. Some contain starting digits, called givens; the others are yours to fill. Each row, column and outlined 3 × 3 box must contain every digit from 1 to 9 once. Nikoli states these same constraints in its publisher rules. [1]

The givens stay fixed. You are not choosing a pleasing arrangement, completing a numerical sequence, or adding the digits to a target. You are finding an arrangement compatible with all the clues at once. If a row already contains 8, writing another 8 in that row is forbidden even if its column looks clear.

Read one cell in three directions

Rows run horizontally and columns vertically. In this guide, r5c5 means row 5, column 5, counting from the upper left. Boxes are the nine regions separated by heavier lines. A row, column or box is also called a house.

The centre cell in the diagram belongs to the middle row, middle column and centre box. The highlighted cells are its peers: cells that cannot share its digit. Notice the overlap. A neighbouring cell may share both its row and box; checking it twice does not create an extra rule.

Keep the rules together

Before considering a digit, inspect the entire row, then the entire column, then the entire box. A digit that survives two checks may fail the third. Do not stop as soon as you find a convenient empty square.

These instructions describe classic Sudoku. A diagonal, cage or irregular region changes the constraints, so read the variant rules before using the same routine elsewhere. Conceptis distinguishes these formats in its rule catalogue. [2]

Keep large answers separate from small candidate notes. An answer says the cell is settled. A note says the digit has not yet been excluded. That distinction prevents an uncertain thought from silently turning into a permanent clue.

Make the first question small

Look at the row below. Eight digits are present: 1, 2, 3, 4, 6, 7, 8 and 9. The missing digit is 5. In a consistent puzzle, the remaining cell must therefore be 5. You do not need to inspect every other empty square before making this deduction.

Now imagine that both 5 and 8 are missing instead. The row tells you the two digits, but not their order. Check each empty cell's column and box. If one cell cannot contain 8, it must contain 5; the other then receives 8. If both digits still fit both cells, leave them unresolved. Having two possibilities is useful information, not an invitation to choose.

Ask about a cell, then about a digit

There are two convenient questions. What digits remain possible in this cell? Where can this particular digit go in this house? The first can reveal a naked single; the second can reveal a hidden single. Stuart's introductory examples explain both views. [3]

Try the second question with 7 in one box. Eliminate occupied cells, rows already containing 7, and columns already containing 7. If exactly one square survives, that is the placement. If several survive, you have found possibilities only. You can write them down or move to a different digit without losing the work.

Avoid three easy bookkeeping errors

Do not alter a given to repair a contradiction. Check your own recent answers and notes first. The fixed clues define the puzzle you are solving.

Do not count only the digits you remember seeing. Look across the full house, including the far end of a row or column. A distant 6 has exactly the same power to exclude another 6 as a nearby one.

Finally, update notes after a placement. If you place 3, remove candidate 3 from its row, column and box peers. Then inspect the changed cells. A fresh answer can leave another cell with one candidate, but old notes can conceal that result.

Finish the sentence before filling the cell

A useful explanation names both the target and the restriction: “r5c5 is 5 because every other digit is excluded,” or “this box's only place for 7 is r3c3.” Saying that a number does not immediately repeat is weaker; it shows permission, not necessity.

Check the short quiz, then open a Standard puzzle and practise one such sentence. If you get stuck, inspect a nearly complete house, switch from cells to digits, or add complete notes in one area. A hint can be useful when it explains the missing deduction. Finishing quickly is optional; understanding the move is the task here.