In computer graphics, 4x4 matrices let you do a rotation and a translation together (among other things). There's the 3x3 rotation block you found later as well as a translation vector embedded in it. Multiplying a sequence of 4x4 matrices together accumulates the rotations and translations appropriately as if they were just a bunch of function applications. i.e. rotate(translate(point)) is just rotation_matrix * translation_matrix * point_vector if you construct your matrices properly. Multiplying a 4x4 matrix with another 4x4 matrix yields a 4x4 matrix result, which means that you can store an arbitrary chain of rotations and translations accumulated together into a single matrix...