If and , then the relation is one-many.
If is called many-one, if and , then the relation is many-one.
If is called one-one , if and .
One-one relation is neither many-one nor one-many.
Less than is both one-many and many-one relation.
Equality is clearly one-one relation.
Fatherhood, Motherhood, Brotherhood, Sisterhood are one-many and many -one relation.
Husbandhood, Wifehood is one-one relation.