I received this question from a reader:
Why is the bottom button of my suit coat supposed to be left undone?
This is what I shared with them:
It may seem like the kind of rule that sprang from a long-standing convention of style whose origins and rationale have been largely forgotten today, such as the stripe on tuxedo trousers or the band on a fedora, but that’s not the case here.
The lowest (bottom button) on a two- or three-button suit coat is generally left unbuttoned for functional reasons: to allow your comfort when you sit, to avoid bunching up when you move your arms, and to accommodate any girth that may have accumulated since your youth.
Alternately, many coats are designed with a single decorative button. These are to be worn open—buttoning the coat could pull and misshape the fabric.