= Ideal hyperbolic quadrilateral
An ideal hyperbolic quadrilateral is the region bounded by four <hyperbolic geodesics> whose four consecutive endpoints are distinct points on the ideal boundary. The ideal vertices lie outside the <hyperbolic plane>. In the <hyperbolic disk>, the four vertices $1,i,-1,-i$ give a regular example: its sides are the interior arcs of the circles of radius one centered at $\pm1\pm i$. Although the sides have infinite length, the quadrilateral has finite <hyperbolic area> $2\pi$ in curvature $-1$.
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