The parallelogram law gives the rule for vector addition of vectors 
and 
.
The sum 
of the vectors is obtained by placing them head to tail and drawing the vector from
the free tail to the free head.
Let 
denote the norm of a quantity. Then the quantities 
and 
are said to satisfy the parallelogram law if
 |
If the norm is defined as 
(the so-called L2-norm),
then the law will always hold.
