Intermediate course · Lesson 4
Locked candidates: pointing and claiming
A candidate can be restricted by the overlap between a 3×3 box and a row or column. Spot that restriction and you can remove the same digit elsewhere—often exposing a new single.
The idea in plain language
Suppose every possible 6 in the top-left box lies in row 2. One of those cells must eventually be 6, so row 2 cannot contain a 6 anywhere outside that box. You do not yet know which boxed cell holds the 6, but you know enough to make eliminations.
Pointing: the box points to a line
Start inside one 3×3 box. Choose a missing digit and mark every legal cell for it. If all those candidates sit on one row, remove that digit from the rest of the row. If they sit on one column, remove it from the rest of the column.
- Choose a box and one missing digit.
- Use crossing givens to find every legal position in the box.
- Check whether all positions share a row or column.
- Remove that digit from the same line outside the box.
- Look for a single created by the elimination.
Claiming: the line claims a box
Claiming runs in the opposite direction. Begin with a row or column. If every candidate for a digit in that line lies inside one box, that digit must occur in the box on that line. Remove it from the box’s other rows or columns.
The logic is identical; only the starting point changes. “Pointing” is box to line. “Claiming” is line to box.
A compact visual pattern
What counts—and what does not
The locked candidates do not have to occupy exactly two cells; two or three candidates work as long as every one is on the same line. The pattern fails if even one additional candidate for that digit exists elsewhere in the box or line you are using as proof.
Only the locked digit is removed. If cells contain candidates 2 and 6, the pattern may remove 6 but says nothing about 2. It also never lets you place a digit directly unless an affected cell is reduced to one candidate.
A reliable search routine
After singles stop, scan boxes one at a time. For each missing digit, ask whether its possible cells line up. Then scan rows and columns with only two or three positions for a digit and ask whether those positions share a box. Keep the search systematic; random staring makes this simple pattern feel rare.
Common mistakes
Eliminating inside the source box
In a pointing pattern, the boxed candidates are the proof, so keep them. Eliminate only from the line beyond the box.
Using incomplete notes
A missed legal candidate breaks the confinement. Recheck all empty cells in the source unit before eliminating.
Expecting an immediate placement
Locked candidates are an elimination technique. Their value often appears one step later as a single or pair.
Practise box by box
Open a Medium practice puzzle with Notes on. When singles stop, choose a digit in each box and ask: “Are all its candidates on one line?” Verify both halves of the logic before erasing anything.
Open practice mode