For a monoid M , we denote by G(M ) the group of units, E(M ) the submonoid generated by the idempotents, and G L (M ) and G R (M ) the submonoids consisting of all left or right units. Writing M for the (monoidal) category of monoids, G, E, G L and G R are all (monoidal) functors M → M. There are other natural functors associated to submonoids generated by combinations of idempotents and oneor two-sided units. The above functors generate a monoid with composition as its operation. We show that this monoid has size 15, and describe its algebraic structure. We also show how to associate certain lattice invariants to a monoid, and classify the lattices that arise in this fashion. A number of examples are discussed throughout, some of which are essential for the proofs of the main theoretical results.