A GAP FOUR is a four with a hole in the middle: two solid pairs on one line, separated by a single empty point. It still threatens five, at exactly one point — the gap. Defenders stare at the solid ends and forget the middle; games are lost on that hole. Build one: black has three stones on row 7 with the left end capped. Make a four whose five-point sits INSIDE the line.
Finish this step to move on.