His subjects either out of respect or fear , followed suit. The trend of leaving the bottom button undone caught on. Sports jackets are usually buttoned the same way but the rules are much more relaxed because it's a more casual style. Your grandfather probably told you about the traditional buttoning rule for a three button jacket — sometimes, always, never.
Here's the bizarre reason you're not supposed to use the bottom button on suits
How To Button Your Suits, Jackets, Vests, Overcoats, & Tuxedo — Gentleman's Gazette
Every craft has it's own lingo, and sewing is no exception. If you want to sew a jacket, you've got collar notches, a collar roll, a collar gorge, and more. Brush up on alllll the key elements who knew there were 37 parts of jacket?! Disclaimer: When it comes to sewing jackets, there is a lot of variation from style to style. Your jacket might not have all these parts, or it might have elements of flair that the diagrams don't. Still, this is a pretty thorough inventory.
The bottom of a man’s suit jacket – button it up or not?
It's fashion gospel for men women are generally allowed to button the bottom button. Where does this tradition come from? The bottom of Edward's waistcoat is undone.
Take, for example, a certain king of Spain who had a lisp. Why not? Because he was too portly to wear suit jackets buttoned at the bottom. Out of respect for their king, English men started leaving their bottom button undone as well.