Italicized sections of this document (if any) are under discussion and have not been finalized.

There are two separate ladders; one for 9x9 boards and one for 19x19 boards. Programs are free to participate in one or both. The ladders are handicap ladders; the number of stones that one program can give the next lower program is explicitly tracked. The meanings of the Stones to next column of the ladder are:

0
Neither program can win as white with 0 stones handicap and 0.5 komi.
1
The stronger program can win as white with 0 stones handicap and 0.5 komi, but not when giving 2 stones handicap with 0.5 komi.
n.5
The stronger program can win when giving n stones handicap with 0.5 komi, but not when giving n+1 stones.

The Stones to next number may have a trailing letter; the meanings of the trailing letters are:

None
The stronger program has won at the indicated level and lost at the next higher level; therefore the indicated handicap is exact.
+
The stronger program has won at this level but not yet played at the next higher level; therefore the indicated handicap is a lower bound.
-
The stronger program has lost at the next higher level but not yet played at the indicated level; therefore the indicated handicap is an upper bound.
?
The stronger program has not played the weaker program.

# Challenge Rules

## Who can be challenged

A ladder participant may challenge the program above to attempt to decrease the number of stones, or the program below to increase the number of stones. In the case where the difference to the program above is 0 stones, the programs would exchange places on the ladder if the challenge were successful. New programs entering the ladder would challenge the "bottom rung" participant at 0 stones, and if successful would challenge successively higher ladder participants (at 0 stones) until unsuccessful. Then handicap challenges would be played to determine the new program's exact position.

If a challenge decreases a handicap, then not only is the handicap above the victorious program decreased, but the handicap below it is increased unless such an increase would violate an established bound. Similarly, increasing the handicap to a lower program via a challenge also decreases the handicap to the program above, unless a bound would be violated. The motivation for this rule is so that if programs A and B have established that A is 3.5 stones above B, and program C beats B, then as C decreases the handicap to A the net handicap from A to B remains 3.5.

## Challenge play

Challenges are played on the IGS at the convenience of the participants. When possible, challenge games are announced in advance on the computer go mailing list. Challenges are single games played at the handicap that the challenger is trying to establish. If the current difference is 0 stones, then the game is played with a 5.5 komi and the defender having the choice of colors. Otherwise, the higher program plays white at the appropriate number of stones with 0.5 komi.

## Time controls

Every participant in the ladder has a noted preferred time control setting for challenge games. The actual time controls used in a challenge game are determined by examining the preferences of the participants and taking the most generous settings of each. For example, if the preference of one participant was 15 minutes pre-byoyomi and 25 minutes / 25 moves byoyomi, and the other participant's was 20 minutes pre-byoyomi and 12 minutes / 25 moves byoyomi, then the match would be played at 20 minutes pre-byoyomi and 25 minutes / 25 moves byoyomi. Note that all byoyomi settings are in groups of 25 moves, since it is awkward to use other numbers of moves for byoyomi settings on the IGS.

## Scoring

There is no absolutely fair way to play matches between programs that use differing scoring systems. The ladder uses a system that tries to be as fair as possible while avoiding any possibility of disputes. Each ladder participant has an indicated preferred scoring system. If a match involves participants whose preferred scoring system agrees, that system is used. If the preferences disagree, then Chinese scoring is used.