Let $_RM$ be a left $R$ -module with submodules $A, B, C$ , and suppose $C \subseteq B$ . Then $$ C + (B \cap A) = B \cap (C+A) $$
This result shows that the submodules of $_RM$ , partially ordered by inclusion, form a modular lattice with $\cap$ as the meet and $+$ as the join.