4 Centres in a Triangle
I = incenter (intersections of 3 angle bisectors)
G = centroid (intersections of 3 medians)
O = circumcenter (intersections of 3 perpendicular bisectors)
H = orthocenter (intersections of 3 altitudes)
I is the center of incircle (inscribed circle of a triangle)
O is the center of circumcircle (triangle inscribed in a circle)
G, O, H are co-linear.