When it comes to what to call other people, it’s easy: I call them what they prefer/ask to be called. If the clarification hasn’t come up, I call them by the name they introduced themselves to me by. So if someone introduces himself to me as “Thomas,” I would not call him “Tom” unless he indicated it was ok. (And I would only ever call him “Tommy” if I knew him really, really well…and even then, just to tease him.) For example, I have a friend named Philip who really dislikes being called “Phil,” so I never call him that. (But I have another friend named Philip who prefers to be called “Flip.” Go figure. ;))
When it comes to what I prefer to be called, it took me a while to figure it out…
My full name is Jennifer, but I have always gone by Jenny: “Jennifer” just always seemed too formal for my personality. However, when I started my current career (6+ years ago), in an attempt to sound more professional I decided that my business cards and voice mail message would say “Jennifer.” I was concerned about my image because I look much younger than I am (at the time I was 27, and looked like a college freshman), and I wanted to be taken seriously in the Big Bad Business World. The first time my father heard my voice mail message he even commented, “I don’t think I’ve ever heard you refer to yourself as ‘Jennifer’ before!”
Well, “Jennifer” didn’t last long. I still preferred to be called “Jenny,” but people would see my business cards or hear my voice mail message and call me “Jennifer” (duh), so when I came to work at my current company 3 years ago I decided it was time to commit to “Jenny.” It worked out, because there is another “Jennifer” here and she does not like to be called Jenny. She prefers Jen or Jennifer, so it has helped to distinguish us (though some people still confuse us: not only did we start within a month of each other, but our last names are both 4 letters long and they rhyme…three years later she still sometimes gets my phone calls!).
I think, too, that by the time I started working here I figured that my personality and quality of work were more important to my professional image than my name or youthful appearance. If someone wants to underestimate me because I go by “Jenny” and look like I just got out of college, it’s their loss.