1998-12-01 last update, 1996-07-07 first day, Robert
Copyright: All rights of the author are preserved according to the
This page intends to ease understanding of The
New Ko Rules. For further important, difficult or extraordinary examples
see Examples 2. Conventions
Definitions for Ko
ko mouths, ko couplings, and ko positions
- On the board are 19 ko stones, 16 ko strings, 13 single ko stones,
3 double ko stones, 16 ko mouths, 5 ko couplings, and 7 ko positions.
- The ko mouths on the board are - denoted by a letter #together# with
the unmarked string adjacent to the letter point - a, b, c-d, e-f, g, h,
i, j, k, l, p, q, r, s and m together with the point right of it and n-o
together with the two points left of n. These ko mouths consist of 2, 2,
4, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, respectively 4 board points.
- The marked strings are strings of ko couplings. 5 ko positions have
a ko coupling. Note: A ko coupling does not belong to a ko position itself.
- The ko positions are - given by the letters #together# with the ko
strings - a-b-c-d-e-f, g-h-i, j-k-l, m, n-o, p-q, and r-s. These ko positions
consist of 12, 6, 6, 2, 4, 4, respectively 4 board points.
- p-q is an example for a ko position with a ko coupling of three strings,
r-s an example for more than three.
Definitions of Ko Types
There are FKs and DKs. A DK may be a DLife or a DDeath. A DLife may
have a ko coupling or not. If a DLife has a ko coupling, then it may be
in one of two states: stable DLife or instable DLife.
DLifes with a ko coupling
- Three ko positions a-b, c-d, e-f-g-h are on the board. Their ko couplings
- c, d are ko mouths of 4 board points.
- All of the strings of all ko couplings have at least two breaths each.
- After W[ba] the B string of the ko coupling of a-b has regained its
second breath. In the other ko positions a second breath can be regained
similarly without repeating the board position.
- In neither of the ko mouths c or d a ko stone could be played, so that
it would not be adjacent to a ko stone of its colour and adjacent to a
ko stone of the opposing colour. (Even after B[cd] such a played ko stone
would not reduce a string of the ko coupling to one breath.)
- Thus all of the ko positions are in a state of stable DLife. (And after
B[cd] the ko position c-d would it be again.) They are DLifes and thus
- Each of the ko stone moves b, c, d, e, f, g, respectively h reduces
a string of a ko coupling to one breath. But in each case the move a, d,
c, g, g, e, respectively e then would transform the corresponding ko position
into a state of stable DLife. Thus after the first move it would be in
a state of instable DLife.
- Stability means coexistence for the time being, instability means irreversable
threat of one player's strings.
- After B[a] the ko position a-b still remains stable.
- Stability is a kind of balance as can be observed for example by playing
a proper MS in the ko position c-d.
- Each of the ko stone moves a, b, c, d, e, f, g, h in a ko position
in a state of stable DLife disturbs a DK due to an reduction of the breath
number of at least one string of the opponent
- A ko position in stable DLife behaves like a seki. It is balanced and
can remain unchanged on the board until the end of the game. Temporary
periods of instability may make life a little bit more exciting.
If a DLife has no ko coupling, then it is a single DLife. The name reflects
its number of ko mouths: one.
- All reasonable known ko mouth types are shown.
- The ko positions a-b, c-d-e, f-g, h, i-j-k, l-m-n are single DLifes.
They do not have ko couplings.
- The positions of o and p do not have single DLifes. In the ko mouth
of o respectively p an infinite ko stone MS can be played. This is also
possible in c-d-e, a ko mouth of the same shape as the ko mouth in p :
4 board points in a row. p has two ko strings, c-d-e not. Since B will
not play at e (and W not at c in the mirrored formation), in c-d-e no infinite
ko stone MS will ever occur in actual play. The ko mouth p has become a
- In the ko position a-b B[baM] is a ko stone MS.
- In the ko position c-d-e a typical MS is W[cdttteMt]. It is used for
- In a-b, f-g, h, i-j-k, respectively l-m-n no infinite ko stone MS exists.
Therefore they are single DLifes.
- i-j-k and l-m-n have triple ko stones.
- B[b], W[c], B[h] disturb one of the ko positions.
- The reader is asked to play through possible MSs.
- A single DLife may behave like a seki and remain on the board til the
end of the game, if it is not disturbed, or else its future is unclear,
when for the possession of the board points of the ko mouth is fought with
the aid of other ko positions.
A DDeath may have a ko coupling or not. If a DDeath has a ko coupling,
then it may be in one of two states: stable DDeath or instable DDeath.
In such a DDeath all cyclical MSs, during which all ko strings of the ko
position are captured, violate the basic ko rule.
DDeaths with a ko coupling
- The ko couplings are marked. The ko positions on the board are a-b,
c-d, e-f, g-h. They all have two ko mouths. Each ko mouth consists of two
- Each of a-b, e-f, g-h also have a ko stone of each colour, at least
one string of a ko coupling with only one breath, and the strings of the
other colour of the ko couplings have at least two breaths.
- Thus the ko positions a-b, e-f, g-h are in a state of stable DDeath.
- The W string of the ko coupling of the ko position c-d has exactly
the breaths of the ko position, i. e. c and d, as its breaths. Thus the
ko position c-d is in a state of instable DDeath.
- B[c] or B[d] transforms the DDeath into a state of stable DDeath. B[cttc]#
or B[dttd]# disturb the DDeath by transforming it from a state of stable
DDeath into instable DDeath.
- W[b], B[f], respectively W[h] give the corresponding ko position ko
strings of only one colour, so it ceases to be in a state of stable DDeath.
Then one W, two B, respectively two W strings of the corresponding ko coupling
have in each case exactly the breaths of the ko position a and b, e and
f, respectively g and h as their breaths. The state is transformed into
instable DDeath. The corresponding moves b, f, h disturb a DDeath.
- In a DDeath a so called eye breath like i or j prevails over a so called
ko breath like c or g. Stability is given during an occurance of a pair
of ko breaths of ko stones of different colour and allows the capture of
so called dead strings.
- The reason of predominance of an eye breath over a ko breath is as
follows: Starting from instability as in the ko position c-d after a ko
capture like B[c] the player of the string of the ko coupling with possible
breaths only being in ko mouths, i. e. W, must answer with a ko threat
B[ct]# elsewhere due to the basic ko rule. Starting from stability as in
the ko position g-h after a ko capture W[h] the player of the strings of
the ko coupling with possible breaths only being in ko mouths, i. e. again
W, must after his opponent's possible answer W[hg]#, which is not prohibited
by rule of basic ko, play a ko threat W[hgt]# elsewhere due to the basic
ko rule. In both cases the basic ko rule forces one and only one player
to make ko threats, while the other side's normal string is alive. So it
is reasonable to define such ko positions as DDeath.
If a DDeath has no ko coupling, then it is a single DDeath. The name
reflects its number of ko mouths: one.
- No ko coupling exists.
- In both ko positions a and b of all adjacent strings to the ko mouths
the B have at least two breaths, what is already enough, while with a B
ko stone in each ko mouth the W have only one breath each and cannot gain
more. Thus a and b are single DDeaths.
- B[atta]# and B[bttb]# disturb a DDeath.
- As can be seen from the examples, in a single DDeath all adjacent strings
to the ko mouth of one and only one player are threatened to become captured
in the moment of ceasing ko mouth existence.
A ko position, that is not a DK, is a FK. In general the nature of FKs
can be described as so called unsettled life and death of involved ko strings.
- Three classical ko positions: a-b-c is a triple ko, d is a basic ko
(this most common ko shape may occur in various surroundings), and e-f
is an eternal life.
- The triple ko has a ko coupling (marked stones). It is not a DLife,
because neither have nor can have after a move all strings of the ko coupling
at least two breaths. It is no DDeath, because it has more than two ko
mouths. So it is a FK.
- In the basic ko a ko stone MS of infinite length can be played. All
adjacent strings to the ko mouth have more than one breath. Thus it is
neither a single DLife nor a single DDeath. It is a FK.
- Also on the eternal life an infinite ko stone MS can be played. The
ko mouth consists of more than two board points. Thus the ko position is
- Ko stone MSs let the ko positions still be FKs.
- In each of the three ko positions the fighting rule applies.
basic ko rule (1)
- The B ko stone is a single ko stone. W[a] captures it by placing a
new single ko stone on the board. By rule of basic ko this new stone must
not be recaptured immediately.
- E. g. W[attatta] with intervening ko threats is possible.
- The beginner's needle in a haystack: All other ko shapes scarcely occur.
The easiest way is to ignore them.
- Note also the fighting rule.
basic ko rule (2)
- This is a double ko, that is in a state of stable DLife. Nevertheless
also the basic ko rule applies.
- B[aa]# and B[abb]# are not allowed due to rule of basic ko.
fighting rule (1) - at b shall be a B stone, at d shall be a
- The eternal life (traditional name) is a FK. Without rules B[acbd]
would be a competently played ko stone MS.
- Application of the fighting rule gives B[attcttbttdtt] as a possible
- In eternal life the burden for the search for ko threats is the same
as in basic ko.
- Note: Before b and d are on the board, b is not played in a FK. After
B[b] the ko position exists and is a single DLife. So W may answer at d.
Then it is a FK, but still B may answer, too. But then for that B's move
the fighting rule applies, i. e. W cannot play c immediately.
- Note: Under Ing ko rules B[attcttbd] is the proper MS.
fighting rule (2)
- The triple ko has a ko coupling (marked) and is a FK.
- After W[a] B must not play on a or c due to the fighting rule.
- W[atta]# must not be answered with a or b, likewise W[attc]# not with
b or c.
- The triple ko is fought, as if it would be a basic ko.
disturber rule (1)
- This triple ko is in a state of stable DLife; it is a DK.
- If B wants to play in the ko position, he may disturb it with B[a].
By disturber rule B as the first to disturb the ko position during a ko
stone MS in it becomes the disturber of it.
- Play may proceed as in B[acbacb]. The ko position of the DK before
the occurance of the disturber, i. e. before the MS, is repeated for the
first time after the MS. By disturber rule the disturber B is then not
allowed to disturb the DK. This remains true while he is disturber or will
be true due to the prohibition rule.
- B needs to tenuki: B[acbacbt]#. This ends the ko stone MS in the DK
and thus B ceases to be disturber by disturber rule.
- B[act], B[acbat] are also conceivable and lead to an earlier end of
the played ko stone MS in the DK and B's disturber title.
- If W wants to play in the ko position, he may disturb it with W[c]
and become disturber. After W[cb] W must not capture any ko stone in the
DK due to rule of basic ko. W[cbt]# ends the ko stone MS and W ceases to
- Note, that after B[ac], B[acba], B[acbacb], or W[cb] the DLife is again
disturber rule (2)
- The ko position is a single DDeath.
- B[a] does not disturb the DK. B[atta]# disturbs it and W becomes the
disturber. After B[attat] W has ceased to be disturber.
prohibition rule (1)
- Three basic FKs are on the board. With more than one ko position on
the board (called string ko) the prohibition rule becomes relevant.
- With B[c] B plays the first ko stone in a fighting ko during the starting
ko stone MS, that may continue as B[cbac].
- So far the configuration of stones on the board position except all
ko positions, that existed before the MS, has been maintained, i. e. repeated,
by each move of the MS. Also the set of board points of the union of all
ko positions has been repeated. Under these conditions no player has played
twice in a fighting ko a ko stone. So the prohibition rule does not apply
- B[cbacb]# and B[cbacba]# are also not prohibited.
- Even B[cbacbac]# is allowed as the exception of the prohibition rule,
that ignores the first ko stone in a fighting ko.
- Finally the prohibition rule hinders W from playing B[cbacbacb]#. Due
to the rules of basic ko and fighting W also must not recapture c.
- If a forth basic ko with a B ko stone in it would exist on the board,
then no rule would prohibit W to capture its ko stone. However, the reality
tells W to play a ko threat B[cbacbact]#. This changes the configuration
of the stones on the union of all board points, that are not part of a
ko position. This virtually lets the prohibition rule forget about all
prior reasons for prohibition. After an answered ko threat both players
might again opt for the trouble of playing through a long ko stone MS followed
by a ko threat etc., e. g. B[cbacbacttcabcabctt..].
- If the players do not like long and fruitless ko stone MSs, they may
restrict themselves to a handy MS like [ctt] and repeat it during the ko
- Now the purpose of the prohibition rule has become clear: String kos
should be played out, as if only one basic ko would be on the board. Only
under special circumstances the players should be interested in a derivation
prohibition rule (2)
- The ko position is a single DLife.
- B[a] disturbs the ko position and B becomes the disturber.
- By B[abMp]# B ceases to be disturber.
- The configuration of the stones on the union of all board points, that
are not part of a ko position, is still the same. The set of board points
of the union of all ko positions is also still the same. Thus the prohibition
rule prohibits B to become again the disturber of the ko position by B[abMpa]#.