The four centres of a triangle
Depending on the context, the 'centre of a triangle' can be thought of in a number of different ways:
First, by constructing perpendicular bisectors of all three sides.
Second, by constructing angle bisectors at all three corners.
Third, by joining up the midpoint of each side with the opposite corner.
Fourth, by dropping a perpendicular line down from each corner to the opposite side (or to where it would extend to if outside the triangle itself.
In each case, the three lines produced will cross in the same place, giving four alternative centres to the triangle. Try moving the corners of the triangle around and see how it affects the centres.