Let be a ring. A non –empty subset of the set is said to be a subring of if is closed under addition and multiplication in and itself is a ring for those operations.

If is any ring, then and are always subrings of . These are said to be improper subrings. The subrings of other than these two, if any, are said to be proper subrings of .

__Theorem__**:** The necessary and sufficient condition for a non-empty subset of a ring to be a subring of are

**(i) **

**(ii)**

__Proof__**:** To prove that the conditions are necessary let us suppose that is a subring of . Obviously is a group with respect to addition, therefore.

Since is closed under addition,

Also is closed with respect to multiplication,

Now to prove that the condition are sufficient suppose is a non-empty subset of for which the conditions (i) and (ii) are satisfied.

From condition (i)

Hence additive identity is in . Now

i.e. each element of possesses additive inverse.

Let then and then from condition (i)

Thus is closed under addition, being subset of , associative and commutative laws of multiplication over addition holds in . Thus is a subring of .