Start with the idea
Start with one number in one 3x3 box. In this example, we are focusing on the number 5. Every possible 5 in box 1 is on the top row. That means the top row's 5 must be inside box 1, so any other 5 in that row, outside box 1, cannot be correct.
Look for this pattern
Look for one number inside a 3x3 box. If every possible spot for that number lines up in one row or column, remove that number from the other squares in that line.
This empty board keeps the puzzle numbers out of the way so the pattern is easier to see.
Example
This technique reviews candidates instead of solving a square right away.
The top row still needs one 5. Because box 1 is the only place in that row where the 5 can belong, remove the other 5 pencil marks from the top row in the boxes to the right.
- Focus on the number 5.
- The only possible 5s in box 1 are on the top row.
- Remove the red 5s from the rest of the top row.
When to look for it
Use it when you are reviewing pencil marks and one number's candidates line up inside a 3x3 box.
How to use it
- Choose one number in a 3x3 box and look at its pencil marks.
- If all of that number's candidates in the box sit on one row or one column, keep them.
- Remove that same number's pencil mark from the rest of the row or column outside the box.
Common mistakes
- Do not choose a square to solve yet; this technique only removes impossible candidates.
- Only remove that one number outside the box on the locked line; leave the other pencil marks alone.