Two groups, one point
A double atari is one stone that puts two separate enemy groups into atari at once — the opponent saves at most one, and you capture the other on the follow-up. The geometry that permits it: both groups draw liberties through the same empty point, their joint bottleneck, so one intersection serves two weaknesses. Scan for it the way you scan for atari (5.2): two weak groups touching a common crossroad is the signature. It rarely needs a second chance — the double atari usually collects a stone immediately, which is why it is the favourite blade inside every fight and every cut. Black to play: find the move that ataris both white stones at once.