What is Caro?
Caro is the form of five in a row played across Vietnam. As in Freestyle Gomoku, a line of five or more stones wins, but with one important exception: a line of exactly five does not win if the opponent has already put a stone at both of its ends.
A five with an opponent's stone at one end still wins, and so does a five that touches the edge of the board, because here the edge does not count as a block. A line of six or more always wins, even with both ends blocked.
The rule matters most when you defend. A four that is already blocked at one end can also be stopped by a stone one point beyond its open end, because the five it could become would then be blocked at both ends. So closed fours are much weaker than in Gomoku, and players watch both ends of every line. The game is played on a 15×15 board, Black moves first, and you can add the Swap or Swap2 opening in the table setup.
Rule changes
| Rule | Standard Gomoku | Caro |
|---|---|---|
| How to win | Standard GomokuMake an unbroken line of exactly five stones. | CaroMake an unbroken line of five or more stones that is not blocked at both ends. |
| A five blocked at both ends | Standard GomokuWins like any other five. | CaroDoes not win when an opponent's stone stands at each end; the game goes on. |
| The board edge | Standard GomokuHas no effect on winning. | CaroDoes not count as a block, so a five against the edge with one end open wins. |
| Six or more in a row | Standard GomokuAllowed, but it does not win. | CaroAlways wins, even when both ends are blocked. |
History
Caro is how most people in Vietnam know five in a row. It is a classic school game, drawn with X and O on squared notebook paper, and it later became one of the most popular board games on Vietnamese game sites and phone apps. On paper the grid has no real edge, so only the opponent's stones can block a line.
The blocked-five rule gives Caro its own character. It is sometimes called Gomoku+, and like the exact-five rule of standard Gomoku and the restrictions of Renju it is one of the ways players have tried to reduce the first player's advantage.