XY-Chain
Follow cells that each have exactly two candidates, with successive cells seeing one another. Each forced second candidate links to the next cell. If the endpoints share a letter and excluding it at one end forces it at the other, remove it from cells seeing both ends.
A valid chain {A,B} → {B,C} → {C,D} → {D,A} means at least one endpoint is A. Other cells seeing both endpoints cannot be A.
Use it on your board
- Choose a row, column or box to inspect. A “unit” means any one of these.
- Check legal candidates against every placed letter. Pencil notes are reminders, not proof: missing or stale notes can hide a possibility.
- Verify every condition in the rule above before making an elimination. A pattern that almost matches is not enough.
- Update the affected notes, then scan for a newly forced letter. A technique often opens the way to a much simpler next move.
Classic and Mega
The candidate rule applies at both board sizes. Classic adds the hidden-line constraint; Mega uses only rows, columns and 5×5 boxes. A larger board has more candidates, so focus on one unit at a time.
These examples explain deductions, not a promise that every generated puzzle requires this technique. In particular, Mega’s launch tiers describe clue density.