The empty triangle is a shape in which three stones form a 90 degree angle. It is also a bad shape, as its stones are too close together.
The three stones in a tiger's mouth also form a 90 degree angle. They are not so overconcentrated, so the tiger's mouth is not as bad as an empty triangle.
(Those who think that the tiger's mouth is a good shape are imagining it in certain configurations of other stones of the opposite color. On the empty board it is overconcentrated. -- Bill Spight.)
Below are other ninety degree shapes formed by three stones of the same color, ordered in increasing Euclidean distance and, presumably, increasing efficiency (more or less). At some point, probably not reached on this page, such shapes become good in themselves as the stones work together.
See big bulge.