If the organisers of a Go tournament in Britain, recognised by the BGA, do not specify otherwise then the Japanese rules are understood to apply except that triple kos are treated as draws, and komi is 6 points.
If the organisers of a Go tournament in Britain, recognised by the BGA, specify that overtime is to be used but omit to say how it is to be implemented, then Canadian (clock-resetting) overtime should be assumed to apply.
Black gives White 6 points komi.
Triple kos and other such repeated positions count as jigo (a draw).
Japanese professional games traditionally use the byoyomi system. When a player has only a few minutes left the seconds are counted down, and any move made in less that a minute does not use up any of that player's time.
In Britain we use the overtime system instead. When a player has used all of their main time allocation, they go into overtime. Overtime is made up of an unlimited number of overtime periods. When a player has used the main allocation, the ﬁrst overtime period begins.
In each overtime period the player must play a speciﬁed number of moves within a speciﬁed time period. Typically, the main allocation might be one hour each, and the overtime periods for each might be ﬁve minutes in which to play thirty stones.
A play (or move) consists in placing a stone on an intersection (or passing) and then pressing the clock. If a player fails to make the speciﬁed number of moves within the speciﬁed interval, they lose immediately.
If a player plays the speciﬁed number of stones within the speciﬁed interval, what happens next? There are various systems.
The next overtime period begins immediately. The clock is set to the time speciﬁed for the next overtime period, and the speciﬁed number of stones are counted out. The next overtime period begins when the opponent next presses the clock.
The player plays on freely until the current allocation of time is consumed, for example until the ﬂag on their clock falls. Then the clock is set to the time speciﬁed for the next overtime period, and the speciﬁed number of stones are counted out.
The player counts out the number of stones speciﬁed for the next overtime period, and starts to play these next turn, while still using the time remaining in the current overtime period. When that allocation of time is consumed (that is, the ﬂag falls) the clock is set to the next allocation of time, which begins immediately.
The player counts out the number of stones speciﬁed for the next overtime period, and adds to the clock the next allocation of time.
The method preferred by the BGA is method 1, Canadian overtime.
Method 3, and particularly method 4, are deprecated by the BGA because of the difficulty of ensuring that they are applied correctly and accurately while at least one of the players is short of time.
With Milton Keynes overtime, the player whose clock must be reset is the player whose move it is. With Canadian overtime, the player whose clock must be reset is the other player. Thus with Canadian overtime, but not with Milton Keynes overtime, the player who is thinking about her move can continue to do so, while the other player resets the clock. This is a reason why Canadian overtime is preferred.
Tournament organisers are free to specify any method of timekeeping that they choose, including methods 3 and 4 above, and methods other than overtime. However if a sufficiently eccentric method is used then the tournament may not be eligible for inclusion in the European ratings system. (Summary of the conditions for a tournament to be included.)
Often, each overtime period will be identical, with the same number of stones to be played in the same interval. For example twenty stones in ﬁve minutes (commonly abbreviated 20/5) or thirty stones in ﬁve minutes (30/5).
Some tournament organisers choose to specify a sequence of differing periods (the same sequence for both players). For example 20/10, then 20/5, then 40/5, then 40/5.