a binary relation is one which is defined for B subset X x X
(x,y) in B <=> xBy
Examples are >, =>, +, ... many others.
1. X=all people, B= kin to.
2. X=all people, B=cousin to.
3. X=R, B= "=>"
4. X=R^2, B= "=>" ("=>" => x1=>y1 and x2=>y2)
5. X=R^2, B= Has greater magnitude