These are the PERPENDICULAR BISECTORS.[br]Their point of concurrency is the CIRCUMCENTER.

Notice that the CIRCUMCENTER is equidistant to the VERTICES.[br]Drag the vertices of the triangle around, and notice that the CIRCUMCENTER is ALWAYS equidistant to the VERTICES.[br][br]Make an acute triangle, a right triangle, and an obtuse triangle. Notice where the CIRCUMCENER is in each case.