This article is from the Go FAQ, by Morten PAHLE firstname.lastname@example.org with numerous contributions by others.
This 'abbreviation' used on Go servers indicates the time limits of
Often, rather than having a time limit for each move or a time limit
for the whole match, many Go matches have two time limits. One is the
'basic' period and the other is the 'overtime', or the 'byo-yomi'
The first part of the 'abbreviation' refers to the basic time limit.
Often on Go servers, the basic time is one minute. You can play as
many or as few stones in this period as you like. As soon as the basic
time ends the first byo-yomi period starts.
The second part of the 'abbreviation' refers to the length of the
Byo-yomi periods require you to maintain a certain average pace: you
must play a certain number of stones (typically 25) within the time
If you play the 25 stones within the byo-yomi period, you get another
byo-yomi period. So if the byo-yomi period is 10 minutes, as soon as
you play stone 25, the clock starts again at 10 minutes. (Even if you
have used less than 10 minutes to play your 25 stones).
If you do not play 25 stones during a byo-yomi period, you lose the
So, if someone proposes a '1/10' match, that is a match with a main
period of 1 minute followed by repeating byo-yomi periods of 10
minutes each. So, when the match starts, the clock counts down from
one minute, then resets to 10 minutes and starts counting down again
In this fashion a 1/5 game with 200 moves cannot take more than 42
minutes: first, each player gets 1 minute, then each player gets 5
minutes to play 25 stones, another 5 minutes for the next 25, etc. If
each player took the maximum time, each player would get one minute
plus 20 minutes (in four 5-minute byo-yomi periods) to play all their
100 stones, for a maximum match time of 42 minutes.
This and other terms used on the go servers are listed and explained