How to solve logic grid puzzles

The two rules that close every case

First: one person per row, and one per column. That is the sudoku half. Placing Ingram in row 2, column C means nobody else can take that row or that column. The empty squares that remain are not decoration — they are the next deduction, because someone still has to occupy them.

Second: the victim takes the last remaining legal square, and whoever shares that room did it. You do not pick a murderer from a list of motives. You finish the placement. The floor plan names them.

Suspectle is not a weapon-and-location grid like some other daily murder-mystery puzzles. There is no separate card for the candlestick or the conservatory. Every suspect gets a row and a column on the same board, and the one left alone with the victim is the killer.

Read the board before you place anyone

A square with furniture drawn on it is taken — a table, a clock, a dye vat. Nobody stands there. A square of bare floor is somewhere a suspect can stand. Rooms are patches of matching floor, with a heavier wall where one room ends and the next begins. The legend under the board names every object and whether you can stand on it, which matters the first time a clue says “beside a club chair” and you need to know which squares count as beside.

Some cases add extra marks. Glyph squares force a person to be alone, or forbid it. A hidden role asks you to name who holds it, not only where they stood. An irregular grid has holes — squares that are not part of the house at all. Empty rows mean fewer people than lines, so some rows hold nobody. Those tags are on the case list; they change what the clues can say, not the two rules above.

Start from the strongest clues

Not every sentence on the clue list is equally sharp. A clue that names a whole room (“Reginald was in the Billiard Room”) cuts the board to that room’s squares. A clue that names a seat (“Reginald was in an armchair”) cuts it to the armchair squares. A clue that someone was alone cuts it to rooms that can hold exactly one person once the other placements are known — and, immediately, it tells you that person is not the murderer, because the murderer shares a room with the victim.

The usual opening is to take two clues about the same person and lay them on top of each other. If someone was in the Billiard Room and in an armchair, you are not looking at the whole house. You are looking at armchair squares inside the Billiard Room. When only one such square exists, that person is placed and you have not guessed. When several remain, that is what Pencil is for.

“Beside” clues come next. They do not name a square; they name a neighbour. Mark every floor square that touches the named object, then let later placements knock the extras off. Direction clues (“north of”, “two squares west”) work the same way: they constrain a line, not a seat, until something else pins the other end.

Pencil marks are not a guess

Switch on Pencil and a tap writes a small note in the square instead of seating someone. Use it whenever a clue leaves two or three legal seats. You are recording what is still possible, the way a sudoku player pencils a candidate. When another placement takes that row or column, erase the notes that just died. Guessing is putting someone down because the square “looks right.” Pencil is the opposite: it keeps the case honest until a square is forced.

Two buttons sit beside that habit. Check names which clue — or which of your own seats — the current board is fighting, without telling you the answer. Hint takes one forced step the engine can already see. Neither is required. They are there so a four-pip case does not turn into a staring contest.

A worked opening: The Locked Library

The easiest case on the list is a fair place to practise the method without being walked to the last square. Four guests — Ingram, Rosalind, Bertram, Reginald — and Constance, the victim, on a 5×5 of Ashcombe Manor. Six clues. Nothing on the board is a glyph or a hidden role. It is pure placement.

Read the list once, then pick the pair that names the same person twice. Reginald was in the Billiard Room, and Reginald was in an armchair. Look at the Billiard Room — on this board it is the whole top row — and see which of those squares is an armchair. That intersection is the first forced seat if only one armchair lives in that room. Place him only when the intersection is unique. If you still see two, pencil both and move on. The guide stops there on purpose: stating the square would be the answer, not the method.

Next, “Bertram was alone.” Alone means his room has nobody else in it, so he is not sharing with Constance, so he is not the murderer. It also means you can ignore any room that already has two open seats you still need to fill with other people. Combine that with “Bertram was beside an armchair” and the remaining armchair squares start to matter as neighbours, not as seats for him. Rosalind beside a club chair, and Ingram beside a longcase clock, are the same shape of clue: mark every legal neighbour, then let the row-and-column rule delete the rest as Reginald’s seat (and later seats) take lines off the board.

When four guests are down, Constance takes whatever legal square is left. Look at her room. Exactly one other person will be in it. That person did it. If two people seem to share it, or none do, a placement earlier in the chain is wrong — hit Check rather than swapping names at random.

When you stall

Go back to unused clues. Every clue in a shipped case is load-bearing; the generator throws away any sentence the solution does not need, so a clue you have not used yet is still doing work. Re-read “alone” and room clues first. Then look at a person who has two clues and ask whether their remaining squares still satisfy both. If the board has gone red, a row, a column, or a named object is already broken — lift the last placement and try the other candidate you pencilled.

Difficulty pips are about chain length, not about hidden tricks. One pip is a short evening. Four is the same two rules with more people, more rooms, and clues that only pay off after three other seats are down. Start on The Locked Library, then step up a pip when the method feels boring rather than when it feels cruel.