Poincaré Disk

Poincaré Disk represents hyperbolic geodesics as Euclidean diameters through the origin or circular arcs orthogonal to the disk boundary. Drag a vertex or enter finite coordinates strictly inside the disk. The displayed triangle has the same angles as its conformal Euclidean drawing, and every nondegenerate hyperbolic triangle has angle sum below 180°; the difference is its angular defect.