Categorizing P2 openings by # of Keystone threats
Posted:
Jul 22, 2021, 12:17 AM
As P2 in pente, one of the most common ways to steal the initiative is to create a tria that forces P1 to make a keystone pair with their K10 stone. (A keystone is a stone that blocks or fills a tessera, which is four same-colored stones in a row separated by no more than one stone of the opposite color. Example: X X X O X, O X X X X O. In both diagrams, the stones marked "O" are keystones.)
This strategy can allow P2 to steal the initiative if P1 relies heavily on the K10 stone, which will disappear if P2 captures the kesytone pair, since P1 is forced to replace the keystone (often with loss of tempo).
This logically leads to a classification of P2 first moves by number of distinct ways that a keystone pair can be created. The more options P2 has, the more P1 has to be careful when devising their opening structure.
SHORT-RANGE OPENINGS (>3 threats)
1...K9 has up to 8 options (sometimes 4 due to symmetry): L9, M9, L8, M7, and their symmetric counterparts.
1...L9 has up to 6 options (rarely 3 due to symmetry). 1...M9 has 5 options. 1...M10 has up to 4 options (often 2 due to symmetry).
MID-RANGE OPENINGS (2-3 threats)
1...N9 has 3 options. It is worth noting, however, that N9 often acts as a short-range opening, because P1's most popular response is 2 N10, which transposes to the K9 opening.
1...N8 has a mere 2 options.
LONG-RANGE OPENINGS (1 threat)
The moves 1...O9, 1...O8 and 1...O7 have exactly 1 keystone threat.
INDIRECT OPENINGS (Zero threats): N10 and M8
These openings do not use keystone threats to steal the initiative. Instead, P2 refuses to reveal their strategy, maintaining flexibility through the first few moves with the hopes of stealing initiative later in the game.
ANALYSIS
I will not be so bold as to say that there is a direct correspondence between the quality of a P2 1st move and the number of keystone threats it makes--to do so would be overly simplistic. One of the advantages of more distant openings is, if played correctly, they force P1 to play moves far away from their desired arena, whereas in short-range openings, P1 can frequently make moves that develop their own plans while thwarting P2's. Nevertheless, the popularity of the short- and mid-range openings at the highest levels is evidence that this is one of the most useful ways to analyze the potential of a P2 opening.
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