# Height of a 2-3-4 tree

Consider the following 2-3-4 tree.

Does this have a height of 3 or 2 ?
If I count from the external nodes ie. the ones with the square boxes then height should be 3.
Not sure if they are to be included in the height as they are represented as NIL.
If they are not to be included then the height of the tree = 2
Any clarification on this.