Read the board with confidence

Cells, houses, coordinates, and what “sees” actually means.

Names are a map

A cell is one square. A house is a complete row, column, or box: any group that needs all nine digits. Rows are numbered from top to bottom; columns from left to right. The label r3c7 means row 3, column 7. It identifies one cell, never a digit.

For exampler1c1 is the top-left cell. r9c9 is the bottom-right cell. Box 1 is top left; box 9 is bottom right.

Seeing means sharing a rule

Two different cells “see” each other when they share a row, a column, or a box. They therefore cannot both hold the same digit. They do not have to touch. Diagonal cells do not see one another just because they make a diagonal line.

For exampler1c1 sees r1c9 through row 1 and r3c3 through box 1. It does not see r4c4.

A target is a question

When a lesson labels a candidate as a target, it means “could we rule this one out?” It is not automatically wrong. To remove it, we still need a reason that covers every possibility in the highlighted pattern. A highlight helps you look; it is not proof.

For exampleIf a 5 must be in cell A or cell B, a target 5 is removable only when its cell sees both A and B.

Check every required connection

A target can see different pattern cells in different ways. It may share a row with one and a box with another. Trace each connection separately. If even one possible location is not seen, that location may still coexist with the target.

For exampleSeeing one endpoint of a two-endpoint argument is insufficient. Ask: what happens if the unseen endpoint is true?

Quick check

A 5 must go in A or B. Your target cell sees A but does not see B. May you erase its 5 using only this fact?

Yes, one connection is enough.
No. If B is 5, the target might still be 5.
Yes, if the target is close to both cells.

Show the answer

No. If B is 5, the target might still be 5. Right. Every possible location must rule out the target before this argument can erase it.

Try it with the interactive lessons Play an easy puzzle

Next, when you are ready: Hidden single.