Flags and chording: when an open number can reveal neighbours

Flags record a conclusion

Holding a covered square adds or removes a flag on this site. With Flag mode ON, a tap also toggles a flag. Open squares cannot be flagged, and an ordinary reveal does not open a flagged square. Use a flag to record a conclusion you can explain from the clues. If you flag every possible mine, the board stops distinguishing proven mines from unresolved choices. The mark itself does not confirm that a mine is present. Before building another deduction on a flag, remember the clue and the unresolved neighbours that forced its location.

The condition for pressing a number again

With Flag mode OFF, pressing an already open number reveals its unflagged neighbours when the number of adjacent flags equals that clue. This action is commonly called a chord. If the counts differ, it does not reveal them. The important limitation is that the operation checks the count of flags, not whether their locations are correct. Two flags around a 2 satisfy the count even if one is on a safe square. The real unflagged mine can then be opened and end the game. Do not add flags merely to make the counter match the number.

Compare correct and incorrect flags at a corner

Put an open 1 in the upper left corner. Call the square to its right A, the square below it B, and the diagonal square C. Suppose other visible clues have already proved that A is the mine. Flagging A and pressing the corner 1 opens B and C safely. Now keep the same mine position but mistakenly flag B. The flag count is still one, so the count test passes. A is unflagged and is included in the reveal, causing an explosion. The corner 1 alone cannot distinguish A, B and C. The separate proof of A is essential.

A short check before a chord

Before pressing, count only flags that touch the number and verify that each is a proven mine. If that proven count equals the clue, no mines remain to be placed among its other neighbours. Use the chord when you can explain that conclusion. At an edge or corner, remember that there are fewer neighbours; there are no hidden squares outside the board. Also check the input mode. With Flag mode ON, tapping an open number is treated as a flag operation and does not chord. Looking at the intended reveal targets first is more useful than trying to click quickly.

Separate flags from the win condition

Winning requires opening every square that is not a mine. You do not need to flag every mine. The total flag count on screen measures your marks, not how many mines you have correctly identified. A warning that flags exceed the mine count is a useful prompt to review them, but a matching total does not certify their positions. A No guessing board can still be lost by chording on incorrect flags. Remove a doubtful mark, recount the local constraints, and use the explanation from a logical hint if needed. Continue from proven squares instead of relying on a tidy-looking total.

Questions and answers

Is a chord always safe when the flag count matches?

No. Matching the count enables the action. Proven correct flag locations are what make the reveal safe.

Do I win by flagging every mine?

You must open every safe square. Flagging all mines is not required.