Almost Locked Candidates Sudoku: Intersections That Eliminate

From Locked to Almost Locked

Start with ordinary locked candidates. When a digit inside a box can only go in cells that also lie on one row or column, it is pointing, and you can remove that digit from the rest of the line. The reverse, claiming, works when a digit on a line is confined to a single box. Almost locked candidates sudoku builds on this by studying the small intersection where a box and a line cross, the three cells they both share.

The word almost is the key. Instead of a digit being fully locked to that intersection, it is locked except for one escape cell. That near-miss, combined with a second near-miss, is what creates new eliminations.

How Almost Locked Candidates Sudoku Combines Two Sets

Picture two digits, each almost confined to the same box-line region but each with one cell of escape. If those escapes interact so that only one arrangement avoids a conflict, you can eliminate candidates that would force an impossible layout. Almost locked candidates sudoku is essentially the locked-candidate version of almost locked set logic, working on intersections rather than whole houses.

• Look at the three cells where a box and a line overlap.
• Find a digit that is locked to that region except for one outside cell.
• Pair it with a second digit in a similar almost-locked state.
• Eliminate candidates that clash with every valid combination.

Why the Eliminations Are Reliable

Like plain pointing and claiming, almost locked candidates sudoku rests on counting, not on assuming the puzzle is unique. Each intersection can hold only so many digits, and once two near-locked digits compete for the same few cells, some outside candidates simply cannot survive. That makes the method safe to use on any grid, including ones where you prefer to avoid uniqueness tricks.

The eliminations are often modest, one or two candidates, but they arrive at moments when nothing else moves, which is exactly when they matter most.

Getting the Feel in Shapedoku

Intersections are easy to picture in Shapedoku, where each digit is a bold glowing shape and the three overlap cells stand out clearly on the dark board. To practice almost locked candidates sudoku, keep pencil Notes on, focus on one box-line crossing, and ask which shapes are nearly stuck there. Smart Notes cleanup keeps the picture tidy as you test ideas.

Work these on Hard and Extreme puzzles, where box-line tension is common. Once you are comfortable with pointing and claiming, almost locked candidates sudoku is the natural next rung, and it will unstick grids that basic locked candidates cannot.