The rules

Every row gets one sentinel. So does every column, and so does every coloured region. The catch is that no two of them may stand next to each other, and that includes diagonally.

Exactly one sentinel in each row.
Exactly one sentinel in each column.
Exactly one sentinel in each coloured region.
No two sentinels touch, not even corner to corner.

The techniques, in the order you will need them

This list is doing double duty. Every entry is a deduction the grading solver actually performs, and the difficulty on a board is decided by how far down the list the solver had to go. A puzzle is only labelled beginner if the first rank was enough, and only expert if the last one was genuinely needed. Learn them in order and you are learning the ladder.

Last cell in a rowBeginner

Every row holds exactly one sentinel. When a row has only one cell left that is not crossed out, that cell must be the sentinel.

Last cell in a columnBeginner

Every column holds exactly one sentinel. When only one cell in a column survives, place the sentinel there.

Last cell in a regionBeginner

Every coloured region holds exactly one sentinel. A region down to a single open cell is solved.

Region locked to a lineIntermediate

If every open cell of a region sits in one row, that region’s sentinel is somewhere in that row - so no other cell in the row can hold one. Cross the rest of the row out.

Line locked to a regionIntermediate

The mirror image: if every open cell of a row lies inside a single region, that region’s sentinel is that row’s sentinel, so the region’s cells outside the row can be crossed out.

Neighbour squeezeAdvanced

Sentinels never touch, even diagonally. When a row’s candidates are packed into a narrow band, any cell in the row above or below that would touch every one of those candidates can be crossed out.

Locked tupleExpert

If two or three rows have their candidates confined to exactly that many columns, those columns are spoken for. Every other row can be cleared of them.

Why you will never need to guess

Every Sentinels board we publish has been through an exhaustive search proving it has one answer, then handed to the technique solver above to be solved a second time. Boards that second solver cannot finish are thrown away before they ever reach the calendar. So the difficulty label is not somebody’s impression of how the puzzle feels. It is a statement about which of these techniques you will have to use.

What every board has to prove →

Where to play

Today’s Daily. One board a day, the same one for everybody, replaced at your local midnight.
Unlimited. Four difficulties, and it does not run out.
Archive. Every Daily we have published. Playing an old one cannot break your streak.