Looking for a puzzle with Hidden Quad…
Hidden Quad
Solve one puzzle and let the hint show the technique at work. Unlimited hints, mistakes do not end the run.
Looking for a puzzle with Hidden Quad…
Four digits can only appear in the same four cells of a unit, so those cells drop all other candidates.
Count the possible cells for every digit still missing from a unit. If four digits have footprints that all fit inside the same four cells, those four cells are reserved for those four digits, and every other candidate in them can go.
Hidden quads are the least common technique in this list. A unit has to be quite empty for four digits to be that constrained, and a unit that empty usually offers something simpler first.
Treat this page as pattern recognition rather than a tool you reach for. Knowing the shape means you will recognise it in the rare grid where it is the only move, instead of concluding the puzzle is unsolvable.
Pick the four digits with the smallest footprints in the unit. Take the union of their cells. If that union has exactly four cells, you have a hidden quad. If it has five, you do not — and no amount of staring will change it.
Four digits must appear in the unit, and all four are restricted to the same four cells. Those cells are therefore fully spoken for, and any fifth candidate in them would have no room.