The three core rules
Standard Sudoku has three core rules. Each row must contain the numbers 1 through 9. Each column must contain the numbers 1 through 9. Each 3x3 box must contain the numbers 1 through 9. The same number cannot appear twice in any one of those areas.
Every square sits at the meeting point of one row, one column, and one box. A number is legal only when it fits all three. If a square's row is missing 8 but its box already contains 8, the square cannot be 8. This is why Sudoku rewards looking in several directions before placing a number.
Rows must contain 1 through 9
A row is a horizontal line of nine squares. When the puzzle is solved, the row will contain 1, 2, 3, 4, 5, 6, 7, 8, and 9 with no repeats. The order does not matter. A solved row could begin with 6 and end with 2. What matters is that all nine digits appear exactly once.
Rows are helpful because they are easy to scan. If a row already has seven numbers, list the two missing digits and test each empty square against its column and box. Many beginner placements come from this simple check.
Columns must contain 1 through 9
A column is a vertical line of nine squares. It follows the same rule as a row: every number from 1 to 9 appears once. Columns often solve a square that looked unclear from the row alone. A row may allow 3 or 8 in a square, but the column may already contain 8, leaving 3 as the only legal answer.
When checking a placement, run your eyes down the column before committing. This one habit catches many errors, especially on phone screens where the selected row may feel more obvious than the selected column.
Each 3x3 box must contain 1 through 9
The thick lines divide the board into nine boxes. Each box is a small 3x3 grid, and each one must contain the digits 1 through 9 once. Boxes are where many Sudoku patterns begin. If a box is missing only one number, the empty square must take that number.
Boxes also interact with rows and columns. If a number can only go in the top row of a box, that fact may remove the same number from other squares in the top row outside the box. This idea is the start of box-line reduction, a useful solving technique.
No guessing for beginners
Guessing can feel tempting when a board slows down, but it usually creates confusion. A beginner should try to place only numbers that can be proved by the rules. If you are choosing between two numbers without a reason, write them as notes and continue scanning elsewhere.
A fair standard Sudoku puzzle has a logical path. Some hard puzzles require deeper techniques, but they still do not require blind guesses. When you practice without guessing, you build skills that carry into harder boards.
Valid puzzle versus solved puzzle
A valid puzzle is a starting grid that does not break the rules and has a proper solution. A solved puzzle is a completed grid where every row, column, and box contains the digits 1 through 9. A partly filled grid can be valid even while many squares are empty.
An invalid puzzle might have two 5s in the same row, or it might have clues that do not conflict at first glance but still make the puzzle impossible. The Sudoku Solver on this site checks for obvious conflicts before trying to solve the grid.
Clues and editable squares
The numbers shown at the start are clues. They are fixed and should not be changed during a normal solve. Empty squares are where you place answers or notes. Sudoku Duck locks clue cells in the online game so the puzzle stays stable.
If you copy a puzzle from paper into the solver, be careful with clues. One wrong copied number can make the puzzle invalid. When something feels impossible, check the givens before assuming the puzzle is too hard.
What counts as a duplicate
A duplicate is the same number appearing twice in one row, one column, or one 3x3 box. The two matching numbers do not need to be next to each other. A 4 in the first square of a row and another 4 at the far end of the same row still break the rule. The same is true for columns and boxes.
When checking a nearly finished puzzle, scan every completed row from left to right, then every column from top to bottom, then each box. If every area contains 1 through 9 once, the grid is solved. This final check is simple, but it builds confidence and catches copied mistakes.