Quadratic elements (six node triangles or eight node quadrilaterals) are composed of two types of nodes, midside nodes and corner nodes. Midside nodes are created automatically when an element is created and are deleted when the element is deleted. The coordinates of midside nodes cannot be edited. Midside nodes are always assumed to be located at the midpoint of the two adjacent corner nodes. When a corner node is edited, the coordinates of the adjacent midside nodes are updated accordingly.
Related Links:
Create_Element_Tools