Claiming

When a digit's candidates in a row or column all fall inside one box, that digit can be removed from the rest of that box.

A real example from a sudoku.ie puzzle. In row 7, the 8 can only go in row 7, columns 1 and 3, which are all inside box 7. That means box 7's 8 must be in row 7, so no other cell in box 7 can be an 8. Remove 8 from row 9, column 3. Yellow cells make the pattern. Red marks show what you can cross out.

What it is

Claiming is the mirror image of pointing. Instead of starting from a box, you start from a row or a column.

If, within one row, a digit can only go in cells that all sit inside the same box, then the row “claims” that digit for that box. The row’s copy of the digit must be in that box. And because the box can only hold one copy, no other cell in the box (outside that row) can have that digit.

The same works for columns. You may also see this called “box/line reduction” or “locked candidates (type 2)”. The names vary, but the idea is the same.

How to spot it

You will need pencil marks. Then:

Pick a row or column and a digit it is still missing.
Find every cell in that row or column where the digit is a candidate.
Check whether they all fall inside one box.

If they do, look at the rest of that box. Any cell outside the row or column you started from, but inside the same box, can have the digit removed.

Rows and columns that are nearly full are good places to start, because the candidates often end up bunched in one box.

A worked example

Look at column 7. It is missing the digit 4.

You check every empty cell in column 7. The 4 is a candidate in only two of them: row 1, column 7 and row 3, column 7. Every other empty cell in column 7 is blocked, either by a 4 in its row or a 4 in its box.

Both of those cells are in box 3, which covers rows 1 to 3 and columns 7 to 9. So column 7’s 4 must be somewhere in box 3.

Box 3 can only have one 4. Since that 4 is going to be in column 7, none of the other cells in box 3 can be 4. These are the cells in columns 8 and 9 of box 3.

Say row 2, column 9 has notes of 4 and 8, and row 1, column 8 has notes of 1, 4 and 5. You can remove the 4 from both. Row 2, column 9 is now left with only 8, so it is a naked single.

Pointing or claiming?

Both involve a box and a line (a row or column) overlapping. The difference is where you start and where you remove:

Pointing: start in a box, candidates line up in a row or column, remove from the rest of that row or column.
Claiming: start in a row or column, candidates sit in one box, remove from the rest of that box.

You do not need to remember the names. Just remember that whenever a digit’s candidates in one unit are all inside another unit, the second unit’s other cells lose that digit.

A common mistake

The usual mistake is removing the digit from the wrong cells. In the example, you remove 4 from the cells of box 3 that are not in column 7. You do not remove anything from column 7 outside the box, because column 7 had no other 4 candidates anyway.

Another common slip is overlooking a candidate further along the line. Before you make any removals, double-check every empty cell of the row or column, including the ones far from the box.

When you will need it

Claiming is part of the Medium toolkit on sudoku.ie, along with pointing and pairs. Expect it on Tuesday, Wednesday and Sunday puzzles. It turns up on Hard and Expert days too, often quietly setting up the next move.