As I've written before, no matter how much you know about something, there's always more to learn.
I recently purchased on eBay a Black Watch (navy with subtle, dark green plaid) odd jacket by Huntington, a defunct Ivy-style clothier about which blogger Heavy Tweed Jacket has often written. The lapel's buttonhole is closed, so I did a Google search about the feasibility of opening it.
I came across a thread on Ask Andy about it; according to some people, the origin of the buttonhole was that there used to be a button on the underside of the other lapel, and a man could pop up the back of his lapel, then pull the lapels across and button them closed to cover the chest. That seems logical, as it would explain why there's only one hole, instead of one on each side.
The lounge suit (what we now call the business suit) originated as casual wear in the late-19th Century. If this story about the buttonhole's origin is correct, I wonder why men didn't just wear overcoats in cold weather? I believe the only overcoats in existence then were frock-style; maybe wearing something so formal over something then considered more casual was considered a faux pas?
I knew that was the purpose of the throat latch, which is the little strip of fabric sometimes attached near the top of an odd jacket's lapel. I wonder how it originated, if its purpose was already served by the jacket's lapel?
I also wonder why the buttonhole didn't disappear when the button under the other lapel did? Maybe because the boutonniere arrived just in time?