First name or nickname, depending on who I’m talking to. Various people know me by different names, so they just use whatever feels best.
Family and friends I grew up with call me Charlie,
Friends and associates from about high school on call me Charles
NOBODY calls me chuck, that was Grandpa and I never answered to it since I don’t really care for it.
Straight Dope and two email accounts are the only places I use the nick/abbreviation chaz or chas
Many Welsh people apparently only use their first name within their family. Their middle name is their public name. My doctor was Dr Huw Llewelyn but his first name was David. My landlord also used his middle name as his public name but was known by his first name to his family.
It’s not clear to me if by being called “Rick” I should have voted for first name or nickname; my legal name is Richard but I am basically never called that except by people reading it off a form, like a nurse or something. It’s not even on my business card.
But maybe I’m wrong and nickname meant something like “Booger” or “The Heap.”
I’ve been ‘Norm’ since my first day at college (alcohol related) in 1990. My mom and god-mother are about the only people who still use my real first name. My dad calls me Norm since we share a first name, it’s just easier.
It wasn’t until years later I discovered that George Wendt (Norm from Cheers) and I shared a birthday.
I go by middle name or it’s contraction because my first name is the same as my Dad’s. I usually sign with my full 3-part name.
As a result; due to the arbitrariness of my employer’s IT and HR departments; at last count I have 4 usable work email addresses! 