What is a mullion is?

Answer:

It's a set of boards between two (or more) windows that are joined together in a group.
It's just trim, it's non-structural in that doesn't support anything above it, it just ties the windows together in the opening.

First answer by Marcucco. Last edit by Marcucco. Contributor trust: 90 [recommend contributor recommended]. Question popularity: 4 [recommend question].