The Rules of Go

Page 07
The Secret of Number 19

19x19 is the largest board for which the center is smaller than the sides. Just imagine placing the black stones on the 3rd line all around the board, and white stones on the 4th line around the board - see Fig 1-1.

Areas

Area in the center equals area on the adges.

Fig 1-1. 19x19 board - edges vs. the center

This will give black the edges and the corners, while it will give white the center! Black territory is 136 points while white's territory is 121 points - so black leads by 15. For comparison, if you try the same to a 21x21 board, the result is 144 for black and 169 for white - so white is leading by 25 points. On larger boards, white would lead even more. Of course, on boards smaller than 19x19, black's advantage in such situation is bigger.

So - now we know: 19x19 is the largest possible board with odd-numbered edges which emphasizes the sides more than the center - in other words, on which the sides are more important. It might be an interesting mental exercise to figure out how the overall Go strategy would have been affected on a 21x21 board - since such board would, by its nature, put more emphasis on the center.