The Rules of Boku
Boku is played on a hexagonal playing board that consists of 80 spaces or fields (Fig.1).
Fig. 1. The Boku board
At the beginning there are no marbles on the board. There are two players; one of them has 36 black marbles; the other player has 36 white marbles.
Starting with Black, the players alternate taking turns and place their marbles on the board, until one player wins by getting five or more marbles in a straight-line, adjacent row.
This rule is in principle the same as in Go-Moku or Renju. But because of the hexagonal arrangement of the fields on the board there are only a maximum of three directions through which marbles can be connected: a horizontal line and two diagonals (Fig.2). In other connection games, which are played on square or rectangular boards, there are a maximum of four directions (eight adjacent fields to any inner field).
Rule 2 (Sandwich-rule)
If player A surrounds two of player B's marbles with two of hers in a direct line then player A has to remove one of those surrounded marbles of her choice and hand it back to player B. On her next move only, player B is not allowed to put a marble back into the spot from which player A just removed the marble.
Fig. 3: The Sandwich rule
Fig. 3 gives an example for the Sandwich rule. If it was Black's turn, she couldn't capture any of the white marbles. If it was White's turn, she could take any of the black marbles except no. 9. If she sets a marble on field no. 14 she could remove either no. 8 or no. 12. If she sets a marble on field no. 5 she could capture any one of the following: no. 3, 4, 7 or 8 ('double sandwich'). Surrounding only one marble or more than two, e.g. by placing a white marble in field no. 13 would not capture any black marble. Also the two surrounded marbles have to be in a straight horizontal or diagonal line, so setting a white marble in field no. 10 would not 'sandwich' the two black marbles no. 8 and no. 9.
The Sandwich-rule is reminiscent of the Ko-rule in Go. There, if player A captured a single stone and player B, by putting another stone back into the place where her first stone was captured from, could recapture player A's stone immediately, player B is not allowed to do this in her next move.
In the official description which comes with Boku, there is a 'suggestion' (rather than a rule) that if one of the players runs out of marbles, the game should be "called a draw". There is no indication of whether the two players could declare a draw by mutual agreement.
Again, the rules give no indication of whether a player may take as long as she wishes before moving or whether she has to comply with time limits either for every move or for the whole game.
Since the board is not rectangular, the standard method of addressing fields through rows and columns using letters and numbers cannot be used without modification. Interestingly, to date, there is no official notation for reproducing game play.