Three nodes in a cartesion coordinate system are used in order to create a triangle element (or short element).
Adjoining Elements share the same nodes.
The line from one node to another is called element edge.
In order to ensure a correct FEM mesh it is not allowed to have a node of an adjoining element on an element edge of another triangle element.
The sequence in which the nodes are selected determines the direction of the normal vector for the triangle element.
The normal vector defines the front side and the back side of an element.
The quality of all elements needs to be checked before a simulation can be started.