Trigonometry/Circles and Triangles/The Miquel Point
In general, four straight lines define four triangles (by omitting each line in turn we have three lines forming a triangle).
The four circumcircles of these triangles all intersect at the Miquel point. This point and the four circumcentres all lie on a circle.
The unique parabola of which the four lines are all tangents has its focus at the Miquel point.