You should also verify that if Black passes at 79, White plays 'b' in
Dia. 4-2-4 and Black loses an entire group, either in the bottom left
or top right. Black cannot afford to pass at 79. Sente is still worth
something at this point.
We see in this way that under area rules, in general there comes a time
at which the player whose turn it is can pass and let his opponent play
instead without having this affect the subsequent result. This point at
which sente ceases to bring any gain is what we want to define as the
point where the last competitive move has been made.
Black 79 in Dia. 4-2-4 is therefore the last competitive move of the
game. As a way of indicating that he thinks the result of the moves
after Black 79 will be unchanged even if he passes, it would be natural
and logical for White actually to pass at this point. If White's
judgment is wrong, his pass will cost him something.
We can accomplish our purpose by formulating the definition of the last
competitive move in the rules in the following way. A player's score is
the number of stones that player has on the board plus the number of
empty points in that player's territory. If the first pass was made by
White, however, then half a point is subtracted from Black's score and
added to White's score. This is rule 7 (the rule of scoring) in area
This rule requires the following modification in the rule about the end
of the game: after the first pass, the game ends when both players pass
in succession. This is rule 6 (end of the game) in area rules III.
The meaning of these rules is clear. When Black or White decides that
sente is no longer worth anything and passes, the immediately preceding
move becomes the last competitive move. If Black passes first he gains
one point, because half a point is not taken from him and given to
White. If White passes first, then White gains one point.
In Dia. 4-2-5 Black wins by five points under area rules II. If White
passes at 80 after Black 79, then under area rules III half a point is
taken away from Black and given to White, making the margin of victory
If White plays 80 by mistake, Black will immediately pass with 81,
making White 80 the last competitive move, and Black will win by five
points. This is White's fault for not passing at 80.
Let's return to Dia. 4-1. If Black can read out that a move at 'a' is
not necessary, then he can pass with Black 5. White 4 becomes the last
competitive move. If Black thinks that a move at 'a' is necessary he
can play Black 5 at 'a,' then White can pass with 6, making Black 5 the
last competitive move. Defining the last competitive move as the move
immediately preceding the first pass in this way gives both players fair
means and fair opportunity to determine when the last competitive move
has been made. They will not have to study the game after it has ended
to decide who made the last competitive move.