Originally Posted by

**Opalg** If p and q are unit vectors (or more generally vectors with the same length) then the points represented by the vectors 0, p, p+q and q form the vertices of a rhombus. The diagonals of a rhombus bisect the angles at the vertices. Thus the vector p+q bisects the angle between p and q.

The other thing I used in the proof is that the condition for two vectors to be perpendicular is that their scalar product ("dot product") should be zero.

Does that help to make things clearer? If not, you'll have to give a bit more detail about what it is that you don't understand.