Platon Sudoku · Sudoku Study

Naked Quad · Step-by-step example

Four cells in one row, column, or box contain only four digits between them. Remove those four digits from every other cell in that region.

This course is adapted from Platon Sudoku App. Read the complete example here, then continue learning and practising in the app.

View on Google Play

Recognize the pattern

Four cells in one row, column, or box contain only four digits between them.

How to find it

Compare four cells with short candidate lists in one region. Their combined set must contain exactly four digits.

What it allows

Remove those four digits from every other cell in that region.

Avoid the mix-up

A quad is not four identical candidate lists. No selected cell may contain a fifth digit, and all four cells must share one region.

Step-by-step example · Row

This example starts from a saved intermediate position. Its candidates include earlier logical eliminations; they are not fresh automatic notes.

Blue: evidence. Green: conclusion. Crossed-out digits: eliminated candidates. R1C1 means row 1, column 1.

Step 1 · Observe the quad

Together, these four cells have only 2, 4, 7, 9 as candidates.

Together, these four cells have only 2, 4, 7, 9 as candidates.

Step 2 · Understand the reservation

Digits cannot repeat in row 9, so these four digits must each occupy one cell.

Digits cannot repeat in row 9, so these four digits must each occupy one cell.

Step 3 · Remove candidates

Remove the marked 2, 4, 7 candidates from other cells in row 9.

Remove the marked 2, 4, 7 candidates from other cells in row 9. R9C4=4, R9C4=7, R9C8=7, R9C9=2

What it allows

Remove the marked 2, 4, 7 candidates from other cells in row 9.

Naked Quad · Remove those four digits from every other cell in that region.

Step-by-step example · Column

This example starts from a saved intermediate position. Its candidates include earlier logical eliminations; they are not fresh automatic notes.

Blue: evidence. Green: conclusion. Crossed-out digits: eliminated candidates. R1C1 means row 1, column 1.

Step 1 · Observe the quad

Together, these four cells have only 1, 2, 6, 7 as candidates.

Together, these four cells have only 1, 2, 6, 7 as candidates.

Step 2 · Understand the reservation

Digits cannot repeat in column 4, so these four digits must each occupy one cell.

Digits cannot repeat in column 4, so these four digits must each occupy one cell.

Step 3 · Remove candidates

Remove the marked 1, 6, 7 candidates from other cells in column 4.

Remove the marked 1, 6, 7 candidates from other cells in column 4. R2C4=7, R7C4=6, R8C4=1

What it allows

Remove the marked 1, 6, 7 candidates from other cells in column 4.

Naked Quad · Remove those four digits from every other cell in that region.

Step-by-step example · Box

This example starts from a saved intermediate position. Its candidates include earlier logical eliminations; they are not fresh automatic notes.

Blue: evidence. Green: conclusion. Crossed-out digits: eliminated candidates. R1C1 means row 1, column 1.

Step 1 · Observe the quad

Together, these four cells have only 1, 3, 5, 9 as candidates.

Together, these four cells have only 1, 3, 5, 9 as candidates.

Step 2 · Understand the reservation

Digits cannot repeat in box 9, so these four digits must each occupy one cell.

Digits cannot repeat in box 9, so these four digits must each occupy one cell.

Step 3 · Remove candidates

Remove the marked 3, 5 candidates from other cells in box 9.

Remove the marked 3, 5 candidates from other cells in box 9. R7C7=3, R7C9=3, R9C8=5

What it allows

Remove the marked 3, 5 candidates from other cells in box 9.

Naked Quad · Remove those four digits from every other cell in that region.

Practice

This course is adapted from Platon Sudoku App. Read the complete example here, then continue learning and practising in the app.

View on Google Play