The parallelogram
method is a little more difficult to describe, but is just as easy in practice as the
headtotail method. The best way to understand this method is to see it performed
visually.

What is a parallelogram? 
Examine the following applet and watch carefully as a parallelogram is
formed by the two vectors being summed.
Applet by Fu Kwun Hwang
Using the mouse, draw two vectors and watch the applet form the
parallelogram. Notice that the resultant vector points from where the tails are
joined to the far corner of the parallelogram.
As I mentioned previously, the headtotail method and parallelogram
method are identical. This is especially obvious when summing two vectors. Using the
above applet, sum two vectors and watch carefully. Do you see how the parallelogram
method naturaly evolves into the headtotail method? (Remember, for the purposes of
summing, a vector can be moved as long as its orientation and length stays the same.)
At this point, you may be wondering why we bother with the
parallelogram method at all. The reason is simple  it is most closely related to the
component metod of summing vectors, which is an important method for finding the exact
length and direction of the resultant vector. 