Platon Sudoku · Sudoku Study
Hidden Quad · Step-by-step example
Within one row, column, or box, four digits have possible positions only in the same four cells. Remove any other candidates from those four cells.
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
Within one row, column, or box, four digits have possible positions only in the same four cells.
How to find it
Choose four digits in one region and check whether all their remaining positions fit within four cells.
What it allows
Remove any other candidates from those four cells.
Avoid the mix-up
The four cells can show extra candidates at first. Check positions for the four digits across the whole region, not only the marked cells.
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 positions
In row 8, 1, 6, 8, 9 can appear only in these four cells.
In row 8, 1, 6, 8, 9 can appear only in these four cells.Step 2 · Understand the reservation
These four digits must each occupy one cell. No other digits fit in these cells.
These four digits must each occupy one cell. No other digits fit in these cells.Step 3 · Remove candidates
Remove the marked 2, 4 candidates from these four cells.
Remove the marked 2, 4 candidates from these four cells. R8C1=4, R8C8=2, R8C9=2What it allows
Remove the marked 2, 4 candidates from these four cells.
Hidden Quad · Remove any other candidates from those four cells.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 positions
In column 4, 1, 5, 6, 7 can appear only in these four cells.
In column 4, 1, 5, 6, 7 can appear only in these four cells.Step 2 · Understand the reservation
These four digits must each occupy one cell. No other digits fit in these cells.
These four digits must each occupy one cell. No other digits fit in these cells.Step 3 · Remove candidates
Remove the marked 2, 4 candidates from these four cells.
Remove the marked 2, 4 candidates from these four cells. R4C4=4, R9C4=2, R9C4=4What it allows
Remove the marked 2, 4 candidates from these four cells.
Hidden Quad · Remove any other candidates from those four cells.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 positions
In box 1, 4, 5, 6, 7 can appear only in these four cells.
In box 1, 4, 5, 6, 7 can appear only in these four cells.Step 2 · Understand the reservation
These four digits must each occupy one cell. No other digits fit in these cells.
These four digits must each occupy one cell. No other digits fit in these cells.Step 3 · Remove candidates
Remove the marked 1, 8 candidates from these four cells.
Remove the marked 1, 8 candidates from these four cells. R2C1=1, R3C1=1, R3C3=8What it allows
Remove the marked 1, 8 candidates from these four cells.
Hidden Quad · Remove any other candidates from those four cells.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 ↗