Nurse Diane and I used to talk about mother's rules. My mother's rules had to do with graciousness and the feelings of one's hostess. Diane's mother's rules were items like, "If you wear a yellow dress, you must have yellow shoes."
Diane's mother's rules have changed with the era. It's still impolite to tell your friend you don't like what she's serving for lunch.
Shoes and purse need not match; in fact it dates us.
Hemlines are all over the place. My personal notion is that knees should be covered. Few really pretty knees are noticed except on the very, very young.
Upper arms should be covered where there is the least hint of flab. I understand trying to stay cool but fashionable and cool are trumped by pale flabby upper arms that need screening from view.
I'm struggling with belts. A glimpse of a belt does indicate a waistline under there somewhere when the flanks are covered with a jacket or best.
Collars: there needs to be a revival of collars. Statement jewelry just overwhelms me. Button down shirts with a strand or 3 of pearls makes a better statement that says something like, "I'm an old dame but I'm chic."