Minesweeper flags and chording: when to open neighbours

A flag records your conclusion that a covered square contains a mine. Chording opens a revealed number's remaining covered neighbours when the adjacent flag count matches that number; it is safe only when those flags actually mark the correct mines.

A flag is a marker, not a new clue

Placing a flag does not reveal the square or change what lies beneath it. The reasoning that justified the flag matters more than the symbol itself. If a 1 has only one covered neighbour, that neighbour is a proved mine and deserves a flag.

By contrast, choosing one of several possible neighbours because it looks suspicious does not prove anything. If you later treat that speculative flag as certain, you can create a chain of deductions that appear consistent but fail on the actual board.

For clear practice, reserve flags for proved mines. If you consider a hypothesis mentally, keep it distinct from the confirmed facts used to decide safe openings.

What makes a number satisfied

A revealed number states how many adjacent mines exist, including diagonals. When you have correctly flagged exactly that many mines beside it, every other adjacent covered square is safe.

Suppose a 2 touches proved mines F and G, plus covered squares A, B and C. The two mines already meet its requirement. A, B and C must all be safe, so they can be opened as a group.

Adjacent to the 2	Status	Consequence
F and G	Proved mines, flagged	The required two mines are accounted for
A, B and C	Covered and unflagged	Safe to reveal
Other neighbours	Already revealed	Add no mines

This is the logical basis of chording. The shortcut executes several safe openings at once after the deduction is complete.

Matching counts can still hide a mistake

Now imagine F is a mine but G is actually safe, while B is the second mine. The interface still sees two flags beside the 2. If the game allows a chord based on that count, it can open B and hit the mine.

The clue never endorsed G. It merely required two mines somewhere in its neighbourhood. Flags are your proposed locations, and a count comparison alone cannot validate them.

This is why checking a chord means asking “how was each flag proved?” rather than only counting the flags. If any adjacent flag came from a guess, the apparent safety of the remaining neighbours depends on that guess too.

The gesture depends on the game

Some Minesweeper interfaces use a combined mouse action or a dedicated control. Touch interfaces may use a tap on the satisfied number or another gesture. Learn the actual control before trying to perform it quickly.

Keep Dig and Flag modes distinct. Selecting a covered square while using the wrong mode can execute a different action from the one you intended, even when your deduction was correct.

Practise on a simple satisfied 1 before using the shortcut repeatedly. Confirm that the surrounding targets are the intended neighbours and that you understand which squares will open.

When manual openings are useful

Chording is convenient when several safe neighbours remain. If only one remains, a direct opening may be just as clear. Manual moves are also useful while learning because they make you name the exact square and its justification.

Do not race to flag every mine before examining safe moves. A newly revealed square may provide more information than another already-known flag. Use flags where they improve counting and enable a justified action.

If a counter decreases as you flag, remember that it typically represents total mines minus your markers, not an independent test of marker accuracy. Reaching zero flags remaining does not prove all unflagged squares safe unless the flags themselves are sound.

A routine for reliable chording

1. Read the clue and include every diagonal neighbour.
2. Count the adjacent flags.
3. Recall the proof for each flagged square.
4. Confirm the count equals the clue.
5. Use the game's chord action on the revealed number.
6. Read the new numbers before choosing the next action.

As this becomes familiar, the checking takes less effort. Speed should come from knowing the relationships well, rather than skipping the justification. Review any mistaken chord by locating the first incorrect flag, since the chord may only expose an earlier reasoning error.

Where to play

Sweeple supports Dig and Flag with taps and continuous dragging, and tapping a satisfied number opens its other neighbours. It is available for iPhone and iPad. Optional Premium adds Auto Flag for already-proved mines; explanatory regular-mode hints can help you understand a flag's basis before using a chord.