Forbidden moves in gomoku
In gomoku with renju rules, Black cannot play certain points. Here is what they are, why they exist, and the one exception.
Why Black has limits
Black moves first, and in five in a row moving first is a big advantage. To balance it, Black cannot make three kinds of moves. White has no limits.
Double three
Black cannot play a stone that makes two open threes at once. An open three is three in a row with free space at both ends, so it can grow into an open four.
Double four
Black cannot play a stone that makes two fours at once, even if one of them is blocked at one end.
Overline: six or more
Black cannot make six or more in a row. For White, six in a row wins as usual.
The exception: exactly five wins
If the move makes exactly five in a row, Black wins, even if the same stone would also make a double three or a double four.
How the game helps you
In gomoku.me, when you play Black, the forbidden points are marked on the board, and a forbidden move is never played: it is refused, you do not lose the game.