The Technique Ladder

Sudoku techniques, easiest to hardest.

In Dayzle the hint and the solver are one engine, so these are the 17 steps it takes to solve a Sudoku board. Open one for a worked example and a clip of the hint teaching it on a real board.

Professor Tile also teaches them one at a time in short video lessons, on YouTube and Instagram. Each technique with a lesson links to it.

Beginner

Naked Single Beginner Video Lesson An empty square where every digit but one is already in its row, column or box.

Pick an empty square whose row, column and box are nearly full, and check which digits are already there. Say Row 3 holds 1, 2, 4, 6 and 9, Column 7 holds 3 and 8, and Box 3 holds 5. The square where Row 3 meets Column 7 can only be 7.

With notes, it is a square that has only one note left. That note is its digit.

Professor Tile's Sudoku School Lesson 1: How Sudoku Works Watch on YouTube

Sudoku: the hint teaching Naked Single on a real board. An empty square where every digit but one is already in its row, column or box. — The app's own hint, on a board from the game.

Hidden Single Beginner Video Lesson A digit that has only one possible square left in some row, column or box.

Turn the question round: pick a row, column or box and a digit it still needs, and look for where that digit can go. If only one empty square there can still take it, the digit goes in that square, whatever other notes the square has.

Say Box 5 still needs a 4, and every empty square in it but one shares a row or column with a 4 already placed. The one square left, say Row 5, Column 6, is 4.

Professor Tile's Sudoku School Lesson 2: Hidden Single Watch on YouTube

Sudoku: the hint teaching Hidden Single on a real board. A digit that has only one possible square left in some row, column or box. — The app's own hint, on a board from the game.

Intermediate

Naked Pair Intermediate Video Lesson Two squares in one row, column or box whose notes are the same two digits.

Say two squares in Row 2 have only the notes 3 and 7. Between them they will hold the 3 and the 7, in some order. No other square in Row 2 can be 3 or 7, so take those notes out of the rest of the row.

Professor Tile's Sudoku School Lesson 3: Naked Pair Watch on YouTube

Sudoku: the hint teaching Naked Pair on a real board. Two squares in one row, column or box whose notes are the same two digits. — The app's own hint, on a board from the game.

Hidden Pair Intermediate Video Lesson Two digits whose notes turn up in only the same two squares of a row, column or box.

Say in Column 4 the notes for 2 and 8 appear in only two squares, in Row 1 and Row 6. Those two squares have to hold the 2 and the 8, so any other notes in them, such as a 5 or a 9, come out.

Professor Tile's Sudoku School Lesson 4: Hidden Pair Watch on YouTube

Sudoku: the hint teaching Hidden Pair on a real board. Two digits whose notes turn up in only the same two squares of a row, column or box. — The app's own hint, on a board from the game.

Pointing Pair Intermediate Video Lesson Inside one box, every note for a digit sits in the same row or column.

Say every note for 6 in Box 1 is in Row 2. The box's 6 will be somewhere in Row 2, inside the box. So no square of Row 2 outside Box 1 can be 6: take the 6 notes out of the rest of the row.

Professor Tile's Sudoku School Lesson 5: Pointing Pair Watch on YouTube

Sudoku: the hint teaching Pointing Pair on a real board. Inside one box, every note for a digit sits in the same row or column. — The app's own hint, on a board from the game.

Box/Line Reduction Intermediate Video Lesson Inside one row or column, every note for a digit sits in the same box.

This is the Pointing Pair turned round. Say the only notes for 5 in Row 8 are in Column 1 and Column 3, both inside Box 7. Row 8's 5 will be in Box 7, so the other squares of Box 7, in Rows 7 and 9, lose their 5 notes.

Professor Tile's Sudoku School Lesson 6: Box/Line Reduction Watch on YouTube

Sudoku: the hint teaching Box/Line Reduction on a real board. Inside one row or column, every note for a digit sits in the same box. — The app's own hint, on a board from the game.

Naked Triple Intermediate Video Lesson Three squares in one row, column or box whose notes, put together, are only three digits.

The three squares need not each hold all three digits. Say three squares in Box 9 have the notes 1 4, 4 9 and 1 4 9. Between them they use only 1, 4 and 9, so those three digits go in those three squares, and every other square in Box 9 loses its 1, 4 and 9 notes.

Professor Tile's Sudoku School Lesson 7: Naked Triple Watch on YouTube

Sudoku: the hint teaching Naked Triple on a real board. Three squares in one row, column or box whose notes, put together, are only three digits. — The app's own hint, on a board from the game.

Hidden Triple Intermediate Video Lesson Three digits whose notes turn up in only the same three squares of a row, column or box.

Say in Row 5 the notes for 3, 6 and 8 appear in only three squares. Those three squares have to hold the 3, 6 and 8, so every other note in them comes out.

Professor Tile's Sudoku School Lesson 8: Hidden Triple Watch on YouTube

Sudoku: the hint teaching Hidden Triple on a real board. Three digits whose notes turn up in only the same three squares of a row, column or box. — The app's own hint, on a board from the game.

Advanced

BUG Advanced Every empty square has exactly two notes except one, which has three.

If every empty square had exactly two notes, you could swap every square to its other note and every rule would still hold, so the puzzle would have two answers. A Sudoku has one, so the board can never end up like that.

When it is one square away, say every square has two notes except Row 4, Column 2, with 2 5 7, count how often each of its three digits appears as a note in its row, its column and its box. The one that appears an odd number of times is the square's digit.

Sudoku: the hint teaching BUG on a real board. Every empty square has exactly two notes except one, which has three. — The app's own hint, on a board from the game.

X-Wing Advanced Video Lesson One digit whose notes, in two rows, sit in only the same two columns.

Say in Row 2 and Row 7 the notes for 4 sit only in Column 3 and Column 8. One of the rows puts its 4 in Column 3 and the other in Column 8, whichever way round. Either way, both columns get their 4 from these two rows, so every other square in Columns 3 and 8 loses its 4 note.

It works the same with rows and columns swapped.

Professor Tile's Sudoku School Lesson 9: X-Wing Watch on YouTube

Sudoku: the hint teaching X-Wing on a real board. One digit whose notes, in two rows, sit in only the same two columns. — The app's own hint, on a board from the game.

Unique Rectangle Advanced Four squares at the corners of a rectangle across two boxes, three of them holding the same two notes and nothing else.

Say the squares at Rows 1 and 3, Columns 2 and 7 form a rectangle across Box 1 and Box 3. Three corners have only the notes 2 6, and the fourth has 2 6 9. If the fourth were 2 or 6 too, the four corners could swap their digits and the puzzle would have two answers.

A puzzle with one answer cannot end like that, so the 2 and 6 come out of the fourth corner, which leaves it 9.

Sudoku: the hint teaching Unique Rectangle on a real board. Four squares at the corners of a rectangle across two boxes, three of them holding the same two notes and nothing else. — The app's own hint, on a board from the game.

Y-Wing Advanced A square with two notes, and two squares it sees that share one note each with it and a third digit with each other.

Say Row 1, Column 1 has the notes 3 5. In its row, Row 1, Column 6 has 3 8; in its column, Row 7, Column 1 has 5 8. If the first square is 3, the square in Column 6 is 8. If it is 5, the square in Row 7 is 8.

Either way one of those two squares is 8, so any square that shares a row, column or box with both of them, here Row 7, Column 6, cannot be 8.

Sudoku: the hint teaching Y-Wing on a real board. A square with two notes, and two squares it sees that share one note each with it and a third digit with each other. — The app's own hint, on a board from the game.

W-Wing Advanced Two squares with the same two notes that do not see each other, joined by a digit with only two places in some row, column or box.

Say Row 2, Column 1 and Row 8, Column 5 both have the notes 4 7, and share no row, column or box. In Row 5, the 4 can only go in Column 1 or Column 5, one in line with each square.

Whichever of those holds the 4, the pair square in the same column cannot be 4, so it is 7. One of the two pair squares is therefore 7, and any square in line with both, Row 2, Column 5 and Row 8, Column 1, loses its 7 note.

Sudoku: the hint teaching W-Wing on a real board. Two squares with the same two notes that do not see each other, joined by a digit with only two places in some row, column or box. — The app's own hint, on a board from the game.

Naked Quad Advanced Four squares in one row, column or box whose notes, put together, are only four digits.

The same idea as a Naked Triple, one size up. Say four squares in Column 6 have the notes 1 3, 3 7, 1 7 8 and 3 8. Between them they use only 1, 3, 7 and 8, so every other square in Column 6 loses those four notes.

Hidden Quad Advanced Four digits whose notes turn up in only the same four squares of a row, column or box.

The same idea as a Hidden Triple, one size up. Say in Box 4 the notes for 2, 4, 5 and 9 appear in only four squares. Those squares have to hold those four digits, so every other note in them comes out.

Expert

Swordfish Expert One digit whose notes, in three rows, sit only in the same three columns.

An X-Wing across three lines. Say in Rows 1, 5 and 9 the notes for 2 sit only in Columns 3, 6 and 8, two or three squares a row. Each of those rows puts its 2 in a different one of the three columns, so all three columns get their 2 from these rows, and every other 2 note in Columns 3, 6 and 8 comes out.

It works the same with rows and columns swapped.

XYZ-Wing Expert A square with three notes and two squares it sees that hold two of them each, with one digit in all three.

Say Row 4, Column 4 has the notes 1 2 6. In its box, Row 5, Column 5 has 1 6; in its row, Row 4, Column 8 has 2 6. If the first square is 1, the box square is 6; if it is 2, the row square is 6; and it may be 6 itself.

So one of the three is 6, and any square that shares a row, column or box with all three, here Row 4, Column 5 or Column 6, cannot be 6.

Sudoku: the hint teaching XYZ-Wing on a real board. A square with three notes and two squares it sees that hold two of them each, with one digit in all three. — The app's own hint, on a board from the game.

Back to Sudoku

Sudoku techniques, easiest to hardest.

Beginner

Intermediate

Advanced

Expert

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