Minesweeper rules: numbers, flags, chording, 3BV

The basics

Open a cell. A number in it shows how many mines are in the 8 neighbouring cells (fewer at corners and edges).
An empty cell means no mines nearby: it and its neighbours open by themselves, sometimes a whole area at once.
Flag a cell you have worked out to be a mine. A flagged cell cannot be opened by accident.
If you open a mine the game ends. When all cells without mines are open you win; flags are optional.

A Minesweeper board: numbers and flags

The first reveal

The first reveal is never a mine: the board is created after your first tap. In No guessing mode it always opens an empty area; in Classic it is simply safe. The clock starts at that moment.

Chording: opening neighbours at once

When a number has as many flags around it as it shows, clicking it opens all its other closed neighbours at once. This is the main secret of speed. But with a wrong flag a chord opens a mine, so flag only when you are sure.

On a phone: a tap reveals, a press and hold (about 0.3 seconds) places a flag with a light vibration. There is also a Reveal / Flag switch at the bottom. Tapping a number with all its flags placed opens the remaining neighbours at once. One finger moves the board, two fingers zoom: these gestures never open a cell.
On a computer: left click reveals, right click flags. To chord (open neighbours at once), click a number with all its flags placed, or use the middle button or both buttons. The Expert board fits the window without scrolling.
Keyboard: arrows select a cell, Space reveals, F flags, H hint, N new game, P pause.

Result figures

Figure	Meaning
Time	From the first reveal to the last safe cell. On the leaderboard the time comes from the server clock.
3BV	The fewest clicks needed to clear the board without flags: one per empty area, plus one per number not touching such an area.
3BV/s	3BV per second: a fair way to compare speed across different boards.
Efficiency	3BV divided by your actual clicks. With good chording it can exceed 100%.