A logic-grid puzzle gives you two or more sets of things — people, places, objects,
times — and a list of clues relating them. Each member of one set pairs with exactly
one member of each other set, and exactly one overall arrangement satisfies every
clue. No outside knowledge is ever required: everything needed is in the clues.
The form is also called a logic problem, a grid puzzle, a matrix puzzle or a
Zebra puzzle, after the best-known early example, and it is the mechanism underneath
murder-mystery deduction games including Deduce.
Why there is a grid
The grid is not decoration and it is not a scoring device — it is a memory aid, and
it is the reason these puzzles are tractable at all. Each cell represents one
proposition: this person was in that room. You mark it ruled out, or
confirmed, or leave it open. A finished board is a handful of ticks in a field of
crosses.
Working without the grid means re-deriving the same facts repeatedly and losing
track of which ones you actually established. Every mark you place is a fact you
never have to work out twice.
The clue vocabulary
Recognising the shape of a clue is most of the skill. The common types:
Direct — states a pairing outright. "Lucinda was in the
conservatory." Mark it and move on.
Negative — rules a pairing out. "The revolver was not in the
study." Usually more valuable than direct clues, because a negative closes a
possibility permanently and there are far more of them to close.
Indirect — identifies someone by a property rather than a name.
"The bespectacled suspect." You must first work out who that is.
Relational — relates two members of the same set. "The two
brothers were in adjacent rooms."
Comparative or ordinal — establishes order rather than identity.
"The doctor arrived before the chemist." These constrain a time or sequence axis.
Conditional — "if this, then that." Constrains without asserting
anything on its own.
Chained — carries two facts in one sentence. Separate them before
marking anything.
How solving actually proceeds
Mark every negative you are given. This is cheap and it is most of the board.
Look for forced cells: when a row or column has a single square left open, that
square is true — and it lets you cross out the rest of its row and column.
Chain your inferences. If the chemist was in the conservatory and the
conservatory held the candlestick, the chemist held the candlestick, even though
no clue said so. This is where the puzzle is won.
Re-read the clues after every breakthrough. A clue that told you nothing on the
first pass often becomes decisive once two other facts are settled.
The detailed method guide works through this at length,
including the mistakes that cost the most time.
Common variants
More axes. Three sets is the classic; a fourth or fifth
(arrival times, motives) multiplies the work rather than adding to it.
Unreliable clues. Some puzzles plant statements that are false,
so establishing which witness lied becomes part of the deduction.
Narrative framing. Murder mysteries are the most popular
wrapper, because "who did it, where, with what" is naturally a three-axis grid
and because it gives the player a reason to care about the answer.
Where to play a logic-grid puzzle daily
Deduce is a free logic-grid detective puzzle with a new case every day: suspects,
rooms and weapons, with arrival times added at the highest rank. Every case is
verified by a constraint solver before publication, so the solution is always
reachable by deduction rather than by guessing — see
how Deduce puzzles are verified. Past
cases stay permanently playable in the archive.