The lower millstone grinds as well as the upper.

Work done in a lower, less visible, or less honored position can be every bit as effective and necessary as work done above it. The saying rejects the assumption that rank alone measures worth or usefulness.

Using the physical arrangement of millstones as a metaphor, it commends people whose roles appear subordinate while not claiming that everyone lower in a hierarchy has identical authority. Apparent rank is no proof that one part carries all the real value.

Use it when defending the competence or importance of someone in an overlooked supporting role, especially where praise or influence has been reserved for those placed higher in a hierarchy. Its tone is figurative and reflective, emphasizing the value of work rather than equal authority.