Grid technique · Advanced
Locked Set
The same idea as a locked pair, extended to three or more items: if k items between them can only reach k possibilities, those k possibilities belong to them.
Why it works
Counting the union, not the individual rows, is what makes this work. Three items don't each need three options — between them, touching exactly three possibilities is enough.
Trigger
Several rows or columns in one mini-grid have each shrunk to two or three live candidates.
Common mistake
Counting the union of candidates incorrectly — missing one column makes a real locked set invisible, and adding one turns nothing into a false pattern.
Example
Mina: llama or cat. Theo: cat or turtle. Rae: llama or turtle. Between them, three people touch exactly three pets, so everyone else is excluded from all three, even though no one person has three options.