Relation is a subset of Cartesian product .

Let us take an example

Let A = {a, b, c, d}

and B = {Aman, Ankit, Baljinder, Chandu, Eklavya}

We define relation as

R = {(x, y): x is the first letter of y, x ∈ A, y ∈ B}

So, our relation R is

R = {(a, Aman), (a, Ankit), (b, Baljinder), (c, Chandu)}

Representing relations with arrow is called arrow diagram

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Now, in our relation

**
Domain
**
= Set of all first elements in R

Domain = {a, b, c}

Note that

Domain ≠ A

**
Range
**
= Set of all second elements in R

Range = {Aman, Ankit, Baljinder, Chandu}

**
Codomain
**
= 2nd set

Codomain = Q = {Aman, Ankit, Baljinder, Chandu, Darshan}