Editing Rules of Go (section)

=====Ko threats=====
The next example is typical of real games. It shows how the ko rule can sometimes be circumvented by first playing elsewhere on the board.

The first diagram below shows the position after Black 1. White can capture the marked black stone by playing at ''a''. The second diagram shows the resulting position. Black cannot immediately recapture at ''b'' because of the ko rule. So Black instead plays 3 in the third diagram. For reasons that will become clear, Black 3 is called a "ko threat".

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w|  r
|  b|  A| bT|  w|  r
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w|  r
|  b| w2|  B|  w|  r
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2|  B|  w|  r
|  b|  b|  w|  w| dr|24}}
|-valign="top"
|White to move
|&nbsp;&nbsp;
|White captures
|&nbsp;&nbsp;
|Black plays away from the ko
|}

At this point, White could choose to connect at ''b'', as shown in the first diagram below. However, this would be strategically unsound, because Black 5 would guarantee that Black could eventually capture the white group altogether, no matter how White played.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2| w4|  w|  r
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2| w4|  w| b5
|  b|  b|  w|  w| dr|24}}
|-valign="top"
|White 4 is an error
|&nbsp;&nbsp;
|Black takes advantage
|}

Instead, White responds correctly to Black 3 with 4 in the first diagram below. Now, contrary to the situation after White 2, Black can legally play at ''b'', because the resulting position, shown in the second diagram, has not occurred previously. It differs from the position after Black 1 because of the presence of Black 3 and White 4 on the board. Now White is prohibited from recapturing at ''a'' by the ko rule. White has no moves elsewhere on the board requiring an immediate reply from Black (ko threats), so White plays the less urgent move 6, capturing the black stone at 3, which could not have evaded capture even if White had waited. In the next diagram, Black connects at ''a'' before White has a chance to recapture. Both players pass and the game ends in this position.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2|  B|  w| w4
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b|  A| b5|  w| w4
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w| w6
|  b|  b|  w|  w|  r
|  b|  A| b5|  w| w4
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w| w6
|  b|  b|  w|  w|  r
|  b| b7| b5|  w| w4
|  b|  b|  w|  w| dr|24}}
|-valign="top"
|White replies to threat
|&nbsp;&nbsp;
|Black recaptures legally
|&nbsp;&nbsp;
|Ko bars recapture at ''a''
|&nbsp;&nbsp;
|Black connects
|}