A Functional dependency is described by X Y between two groups of attributes Y and X that are subsets of R specifies a constraint on the possible tuple that can make a relation state r of R. The constraint is for any two tuples t2 and t1 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This seems the value of X element of a tuple uniquely calculates the value of element Y.