What are the body diagonals of a cube and how do you calculate them?

[DH]

Can some one tell me what are the diagonals of a cube?
Picture is better

 

[mezarashi]

Firstly, think of a diagonal of a square.

A---------------B






C---------------D

It would A to D, D to A, C to B, or B to C. With a three dimensional cube, similarly, it is from one corner to the other "extreme" corner. That is the corner most farthest away. So if you are at the near-top-right corner, the diagonal would be a line to the far-bottom-left corner.