How Othello Works: Flipping
Othello (Reversi) is played by trapping the opponent’s discs between your own and flipping them to your colour. The puzzles here ask for the single best move, so understanding flipping first makes them much easier. New to Othello? Begin with these diagrams.
Place a disc so that a straight line of the opponent’s discs (○) is trapped between your new disc (dashed) and another of your discs (●). The trapped discs flip to your colour — here, dropping black on the right flips the two white discs (ringed) to black.
A single move can sandwich lines in several directions at once. This move flips the white disc above (vertical) and the one to its left (horizontal) together. Every flippable direction flips.
Flips happen along diagonals too, not just rows and columns. You may only play where you would flip at least one disc — never where nothing is sandwiched. Tsume Othello puzzles use this rule to find the single best move (the one that maximises your disc lead).