SOS (paper-and-pencil game)

SOS is a paper and pencil game for two or more players that is played on a grid of squares, most often a simple square grid drawn freehand before play begins. On each turn a player writes either the letter S or the letter O into any empty square, and a player who thereby completes the sequence S, O, S reading horizontally, vertically or diagonally scores a point and, in the standard rules, takes another turn. Play continues until every square on the grid is filled, at which point whichever player has completed the most S O S sequences wins, with games ending in a tie possible under some scoring rules. The game is closely related in spirit to tic-tac-toe and to dots and boxes, sharing their combinatorial, perfect information structure in which no element of chance decides the outcome and every move is visible to both players, and it is formally classified in game theory terms as a zero sum sequential game of perfect information. Its minimal equipment of only paper, a pencil and a drawn grid has made it a durable pastime for passing time with a single opponent using nothing but writing materials.