The triangle choke (sankaku jime) is a blood choke applied with your legs. From guard, you swing one leg across your opponent's neck, lock your other leg behind it in a figure-four, and use your opponent's trapped arm to close the choke against one side of their neck. Your leg closes the other side. The combined pressure cuts blood flow through both carotid arteries.
Because the choke uses leg power rather than arm strength, it is accessible for practitioners of all sizes. It also links naturally to the armbar and omoplata — a defended triangle creates openings for both, making the threat system hard to break down.
The triangle is most commonly applied from closed guard, but versions exist from mount, side control, and even standing.
For full details on the guard version, see Triangle Choke from Guard.