High Quality Content by WIKIPEDIA articles! In geometry, a tangential quadrilateral is a convex quadrilateral whose sides all lie tangent to a single circle inscribed within the quadrilateral. This circle is called the incircle. Because tangential quadrilaterals can be drawn surrounding or circumscribing their incircles, they are also sometimes called circumscribed quadrilaterals or circumscribable quadrilaterals. Another name for the same class of quadrilaterals is inscriptable quadrilaterals.