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