The center of the incircle, called the incenter, can be found as the intersection of the three internal angle bisectors. The center of an excircle is the intersection of the internal bisector of one angle (at vertex formula_1, for example) and the external bisectors of the other two. The center of this excircle is called the excenter relative to the vertex formula_1, or the excenter of formula_1. Because the internal bisector of an angle is perpendicular to its external bisector, it follows that the center of the incircle together with the three excircle centers form an orthocentric system.