All boards are constructed so that guessing is never required!
Tiling
Asking once marks what to look at; asking again names the reasoning.
Click to uncover. Right-click to plant a flag. Clicking a number that already has its full quota of flags beside it uncovers everything else it touches.
Uncovering a mine shows it for a moment and covers it again; the board is never lost, only given up on. A board finished without touching a mine and without asking for a hint sets a best time — any other is played for its own sake.
Difficulty
Each level stands on a ladder of reasoning. Easy asks only one-glance rules — counting, full groups, boxes read like numbers. Medium hides some of the numbers and asks two-fact rules besides: overlaps, bottlenecks, sealed pockets. Hard hides numbers too and needs the deep ones — crossed numbers, budgets, the endgame against the mine counter. Harder hides every number the board can spare and hunts for boards that need as many different kinds of reasoning as it can find, so a large one takes a few seconds to deal. A cell that never shows its count still proves itself safe, and every board on every level can be finished without guessing.
Strict
With this on, uncovering a cell that the board does not yet rule out as a mine finds a mine there. Only cells you can actually prove safe are safe, so guessing never pays. Every board can still be finished, and boards played this way set no best time.
Show neighbours
With this on, resting the pointer on a number lights the cells it counts — which a tiling of pentagons does not make obvious by looking.
Loose box clues
With this on, a box that would hand over its whole contents — a nought, or a box needing every cell it has left — may say “at least” or “at most” instead of its exact count, so its ground stays covered and has to be reasoned for. Only the box laws are touched.
Ruleset
Board
Cells across
Cells down
Mines