L1 Foundation
Last Remaining
By checking where a number already appears in neighboring rows and columns, you can determine that it can only fit in one cell of a box.
What to look for: Pick a number, scan its placements in surrounding boxes. Crossing out occupied rows and columns often leaves exactly one possibility.
Pro tip: This is the bread-and-butter technique. Master cross-hatching and you will solve most Easy/Medium puzzles.
WSRF Zone Signal
HOT digit with cross-hatch narrowing to single box position.
● WSRF Shape: Zone Lock