An asterism in Hercules, formed from 5 stars. Not one of the 88 constellations.
A lopsided square that is unremarkable in itself, but it is how you find M13, the finest globular cluster in the northern sky, sitting on its western edge.
The Keystone is centred near right ascension 16.9 hours, declination +35°. It sits well north on the sky and is comfortably placed for northern hemisphere observers.
The International Astronomical Union recognises 88 constellations, and they divide the whole sky between them like countries on a map. The Keystone is not one of them — it is a pattern picked out of Hercules. That makes it an asterism: a shape people recognise and name, with no official standing whatsoever.
The distinction matters less than it sounds. Asterisms are usually easier to find than the constellations containing them, which is exactly why they survive.
SkyRef can draw the Keystone in its real position for wherever you are — switch on the asterism layer and it appears dashed across the constellations it runs through.