1. Conversion Go is a game for two players, Black and White, who alternately put stones of their color on the board. Once placed, stones do not move. They are, however, subject to conversion during the course of the game, as well as to automatic conversion at game's end, prior to counting the score.
2. The board is a 16x16 grid (counting the points rather than the squares), as shown in Diagram 1. At the beginning the board is empty, with play taking place on the points.
Diagram 1 --- the board for playing conversion Go
3. A corner point is considered to belong to both edges that meet there.
4. It is permissible to pass. If both players pass consecutively, the game concludes.
5. It is not permissible to "mirror" the opponent's moves ten or more turns in succession.
6. The beginning of the game is governed by a "refined" pie rule. Player 1 plays two moves for Black and one move for White. Player 2 then decides which side to take, with White to move. Note that the standard pie rule is encapsulated within the refined rule, because it is permissible to pass a turn. (I.e., Player 1 could play one move for Black, then pass for White, then pass for Black.)
7. When a solidly connected group of stones loses all of its liberties --- orthogonally adjacent vacant points --- it is "converted," meaning that it is removed from the board and replaced with stones of the opponent's color. (If you happen to have a bunch of Othello pieces, that would be more convenient since you could flip them.) There is one important restriction on the conversion rule: If the group touches two or more sides, it is considered immune --- permanently exempt from being converted.
In Diagram 2 are shown some examples of conversion and some examples of immunity. At the upper middle Black can play on the point marked in orange, taking away the white group's last liberty and thereby converting the group to black. On the left White can play on the point marked in orange, causing the six-stone black group to be converted. In the lower left White has a group which is immune; it touches two sides --- left and bottom --- and thus is not converted despite being without any liberties. (To use Go lingo, the group is unconditionally alive.) Observe the large white group running down the right side. All of its liberties are occupied by black stones, but it lives because it touches two sides of the board (top and bottom). Consider the situation at the lower right. Because a corner point is considered to belong to both sides that meet there, the lone black stone is immune from conversion! It's tempting to conclude that it is wise to play a stone on the very corner. But in the early stages of the game it would be a weak move, the reason being it has very little influence to the outside. A stone three or four points diagonally inward, though not yet explicitly safe, is much more powerful.
Diagram 2 --- conversion and immunity
8. Special situations will occasionally arise, requiring special consideration/rules. One such situation is "suicide" --- a move that takes away the last liberty from one's own group (without also taking away the last liberty of an opposing group). In conversion Go suicide is legal and results in one's own stones being converted. Another special situation is "simultaneity," which is when both players' groups are reduced to zero liberties at the same time. The ruling is that only the group belonging to the player not making the move is converted. The last special situation is known in Go as seki, or local impasse. It arises when both players' groups have been reduced to the same two liberties. Neither player will play on either liberty, because the other player would then be able to play on the other liberty, thus making a conversion in accordance with the ruling on simultaneity. The two points will therefore remain empty, counting as territory for neither player.
In Diagram 3 there are some examples of these special situations. In the upper right, if Black plays on the point marked in orange, it is suicide: The resulting five-stone group is converted to white. At the upper left is an example of simultaneity --- White can play on the point marked in orange and convert two black stones. (White is taking away his own group's last liberty, but because he is in the process of converting his opponent, his own group is not converted.) Consider the odd situation at the right. Black can play on the point marked in orange, thus converting the four white stones; however, the larger black group that results now has no liberties, so it is immediately converted to white. (This amounts to a complex instance of suicide.) In the lower left is a seki: neither player is motivated to play on either of the two points marked in orange, from fear of being converted on the next move. So the two points will remain empty and do not count as territory for anyone.
Diagram 3 --- special situations
9. The game concludes when both players pass consecutively. (At this stage there are no more profitable moves to be made.) Any stones that cannot escape conversion are now automatically converted. (Or you can simply remove them from the board without replacing them; the identical score will result.) Each player then figures his score, which is the number of his stones on the board plus the territory --- vacant points --- they surround. Note that the total score will always be 256 minus two for each instance of seki.
Diagram 4 shows a position just after both players have passed. The following white stones are doomed: the three-stone group on the top (at F16, G16 and H16); the two-stone group on the left (at A11 and A10); and the loose stones at G9 and L4. The following black stones are doomed: the three-stone group on the bottom (at G2, H2 and H3); and the lone stone at A15. Observe that there are two instances of seki on the board, denoted by the four empty points marked in orange. Black wins by a score of 136 to 116.
Diagram 4 --- both players pass
It may seem that conversion Go, because it involves stones which are converted and re-converted, cannot be played neatly with pen-and-paper. But strangely it can. The reason is that the color of any "X"ed stones is always clear --- it is the color of the living formation which ultimately encloses them. Diagram 5 shows some examples of this "ultimate enclosure" principle. In the upper right the color of the "X"ed stones --- both black and white --- is understood to be black, because that is the color of the living formation which encloses them. (This implies a single solid black group, which you'll note touches two sides and thus can never be converted.) In the upper left of the diagram, a living formation of black stones surrounds six "X"ed white stones and one "X"ed black stone; the color of all seven is understood to be black. At the bottom, White has a large living formation which encloses a jumbled assortment of "X"ed black and white stones, all of which are clearly white.
Diagram 5 --- ultimate enclosure
Diagram 6 shows a completed pen-and-paper game, which White has won by a score of 171 to 85. Some "X"ed stones were converted during the course of the game, and some were automatically converted at the end of the game (because they could not escape conversion); as an exercise, you may wish to investigate for yourself which are of necessity which. Note that the color of all "X"ed stones is unambiguous.
Diagram 6 --- a completed pen-and-paper game