The 1-1 pattern
When all the hidden cells of one 1 are also cells of the 1 next to it, the extra cell of the second 1 is safe.
The first 1 has two hidden cells and one mine between them. The second 1 touches the same two cells and one more. Its only mine is already among the shared cells, so its extra cell cannot be a mine. The pattern often starts at a wall: the edge of the board or open cells.
How it works
Two 1s stand side by side. The first touches only two hidden cells (yellow); the second touches those two and more.
Your turn
Each board is a real position from a game. Tap or click to open; right-click, long-press or switch on Flag to place a flag. If you are not sure, take a hint: the first one only shows where to look.
Which hidden cells can you be sure of?
How the exercises were chosen
A computer checked every board: in what you see, no number on its own proves anything, and comparing two numbers proves only what the 1-1 pattern does.