Block - a running game
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The mechanism of a 'running game'.
In a running game, one and sometimes both players' moves are forced. Here it develops from a hypothetical situation without the context of an actual game. In actual play the outcome of course largely depends on the overall game position.
Running games don't always develop on the two lowest lines either. The initial position on this page is itself in part a result of a running game of sorts along the p-line and o-line. However, the term is usually kept for battles along the first/second, or the second/third line.