In mathematics, a relation on a set may, or may not, hold between two given set members.For example, "is less than" is a relation on the set of natural numbers; it holds e.g. between 1 and 3 (denoted as 1<3) , and likewise between 3 and 4 (denoted as 3<4), but neither between 3 and 1 nor between 4 and 4.As another example, "is sister of" is a relation on the set of all people, it holds e.g. between Marie Curie and Bronisława Dłuska, and likewise vice versa.Set members may not be in relation "to a certain degree", hence e.g. "has some resemblance to" cannot be a relation.