1Definition: Locked Candidates are similar to Hard-Candidates in that they lay claim to candidates on a Lane within a Block, and point out that other Blocks cannot contain the candidate on that Lane.
2Look For: Candidates which are confined to one Lane of a Block ("pointing") or where all the candidates in a Lane exist in only one Block ("claiming").
3Consequence: For claiming Locked Candidates, the candidate can be erased from any other cells in the Block outside the Lane. For pointing Locked Candidates, the candidate can be erased from any other cells outside the Block on the same Lane.
4Why it Works: When a candidate can only go in one Lane of one Block, it cannot appear outside of that restriction.
Locked Candidates: Pointing in yellow; Claiming in green.
6In our example, there is a Locked Candidate 5 in Column A for Block 4. (i.e. in column A, all of the 5's appear only in Block 4.) Since there are other 5's in the Block, but no 5's outside the Block on Column A, we know that this is a claiming type. (A good way to remember this is that the pointing type points to candidates outside the Block.) As there are no 5's outside of the Block in Column A to point to (or erase), the claiming type of Locked Candidate erases 5's inside the Block: on B4 and B6.
7We find a pointing Locked Candidate in Block 6. There we see that candidate 3 only appears in row 5. There are no other 3's in the Block to erase, but this pointing Locked Candidate points to 3's outside of this Block on this Lane (row 5.) They cannot be 3's because that would leave no place for a 3 to go in Block 6. Therefore, we can erase these 3's.
8Your Turn! There are two more Locked Candidates in Block 6: one for 5's and one for 9's. Both are pointing type. In order to continue this tutorial, erase the 5's and 9's that are pointed to.
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