If A is a subset of B but A ≠ B, then A is called the proper subset of B, and B is called the proper superset of A. This relationship is represented as below

A ⊂ B ( A is a proper subset of A )
B ⊃ A ( B is a proper superset of A )

Examples ⇒

{ x² | x ∈ N } ⊂ N ( not all natural numbers are squares )
{ 1, 2, 3 } ⊂ { 1, 2, 3, 4 }
{ 1, 2, 3, 3 } ⊂ { 1, 2, 3, 4 }