: 𝑹𝑴⁄is a field if and only if 𝑴 is a maximal ideal of 𝑹.

Theorem: Let 𝑹 be a commutative ring with unity, and let 𝑴 be an ideal of 𝑅. Then 𝑹𝑴⁄is a field if and only if 𝑴 is a maximal ideal of 𝑹.