SOS is one of the great classroom games: it needs nothing but squared paper and two pencils, a round takes five minutes, and the rules fit in a sentence — yet it rewards real thinking. If you've never played, or you've played but argued about the details, here is the complete rulebook.
What you need
A square grid and two players. On paper, people usually draw anything from 4×4 to 8×8 — bigger grids make longer games. On RivalPit the board is drawn for you, scoring lines appear automatically, and you can play the computer, share a device, or send a friend a challenge link.
The rules
- Players alternate turns. There are no separate letters per player — this is the key rule newcomers miss. Both players may write either an S or an O on any empty square, and choose freshly every turn.
- The goal is to complete the sequence S-O-S in a straight line — horizontally, vertically, or diagonally. The three letters must be adjacent squares in a row.
- Completing an S-O-S scores one point. Mark the line through it (traditionally each player uses a different colour so the score stays readable).
- Scoring earns an extra turn. After completing an SOS you move again immediately. If that move completes another SOS, you keep going — there is no limit to a scoring chain.
- One move can score more than once. If a single letter completes two S-O-S sequences at the same time (say, one across and one diagonal), it scores two points — and still earns the extra turn.
- The game ends when the grid is full. Most points wins. Equal points is a draw.
A worked example
Suppose a row reads S, empty, S. If it's your turn, you write an O in the gap: S-O-S, one point, move again. Now suppose a column reads S, O, empty. An S at the bottom finishes it. The dangerous flip side: if you create "S, empty, S" and it isn't your extra turn, you've simply handed your opponent a free point — which is why good SOS play is mostly about not creating finishable patterns. (Our strategy guide goes deep on that.)
Frequently argued points, settled
- Can I pass? No. You must place a letter every turn, even when every square is dangerous. Forced moves are where the drama lives.
- Do diagonals count? Yes, both directions. Half of all missed scores in casual play are diagonals.
- Can an S or O be reused? Yes — letters can participate in multiple S-O-S sequences over the course of the game, as long as each exact three-square line is only scored once.
- Who goes first? Flip a coin. On even-sized boards, going second is a slight structural advantage in expert play — alternate the starting player over a series.
Where SOS comes from
SOS belongs to the family of paper-and-pencil "connection" games that flourished before video games — cousins of noughts and crosses and dots and boxes. Its exact origin is undocumented (schoolyards don't keep archives), but it spread through classrooms worldwide in the 20th century, and its name simply borrows the famous distress signal — three letters everyone already knew how to spell. It even appeared as a puzzle subject in recreational mathematics columns, because the "safe move" question turns out to be genuinely interesting combinatorics.
Variants to try
- Board sizes. 4×4 is a quick tactical scrap; 8×8 is a positional battle. RivalPit supports 4×4 through 8×8.
- First-to-N. Instead of filling the board, first player to reach an agreed score wins — good for very large paper grids.
- Team SOS. Four players, two teams, alternating seats — chaotic and loud, exactly as a classroom game should be.