Lesson 7 of 9
Pointing pairs
A digit cornered onto one line inside a box clears that line in the two neighbouring boxes.
What this grid does
- Starting digits
- 25 of 81
- Deductions to solve
- 60
- Uses of Pointing pairs
- 1
- Digits placed before it is needed
- 18
- First needed at
- deduction 22, in box 5 (R5C5 and R6C5, on the 7)
- Also calls for
- last free cell, cross-hatching, naked single, hidden single and naked pairs
Measured on this grid by the same solver the hint button uses — how these puzzles are chosen.
Why this one is worth drilling
Only about 1 puzzle in 14 needs it. That is the whole reason this lesson exists: you cannot learn a deduction a random grid almost never asks you for, so it has to be set deliberately. That figure is measured rather than guessed: 7.3% of solvable 22-clue grids over 4,000 digs.
The jam here is usually that you are working inside one unit at a time. This deduction lives between two units — a box and a line — and no amount of looking at either one on its own will produce it.
Where it sits in the nine
Seventh, and the first technique that reasons across two units at once rather than inside one. That is a real change of gear, and it is why it waits until the single-unit techniques are solid. It also sets up lesson eight, which is this same idea run in the opposite direction.
Working it
Work box by box, and for each digit the box still needs, check whether its remaining squares happen to share a row or a column. That is the entire search. Then look outside the box along that line: the eliminations land away from the squares you reasoned about, which is the part that feels wrong the first few times. The eliminations are the easy part to get wrong. Having found that a digit is confined to one line within a box, the digits you remove are the ones on that line outside the box — never the ones inside it, which are the possibilities you just proved. Reversing that is the most common error here, and it corrupts a grid quietly enough that the contradiction only surfaces much later.
The full explanation of pointing pairs covers the reasoning with diagrams.
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