Dear editor-on-call,
I always forget when certain words should be capitalized, like sir (Sir?). Can you help?
Sincerely,
Case sensitive
Dear Case,
My quirky post title is a good mnemonic device: Don a cap[ital] if you're proper. In other words, capitalize proper nouns, but leave common nouns lowercase.
A proper noun is a NAME. For the most part, this is pretty simple to understand. Anne Shirley loves Gilbert Blythe, not gilbert blythe. (She might IM with gil_blythe, but I digress).
Trademarks are names (Barbie, Kleenex, Lycra), weekdays and months are names (Monday, September), artistic work titles are names (The Shining, Evita, Mona Lisa), specific places have names (Yosemite, London, Lake Country, Serengeti Plain), specific events have names (Lycoming County Fair, Little Bears Fun Run, Easter, Rosh Hashanah).
The tricky thing is when common nouns behave like proper nouns, or transform as part of a compound proper noun.
Let's look at your example, "sir." It's one of those courteous words waiters use when talking to men, hoping for a big tip: "And what will you have tonight, sir? May I recommend a wine to pair with that, sir?" That's the most usual use in our culture.
But once upon a time (and once upon today in certain social circles), there existed men of noble rank whose name was always preceded by a "sir," and the title was considered part of the name. Therefore, the common noun shifts to proper noun when it becomes part of a name. (You picking up a theme here?)
So, for example, your historical (or fantasy or upmarket) fiction might have sentences like this:
Sir Wallace stomped into the house, furious. "Where is my son?!" he bellowed. "Where is Sir Reginald?"
His servant bowed low. "I know not, m'lord, sir. If you please, sir, I have not seen Sir Reginald since breakfast."
Generally, sir will be lowercase unless paired with the nobleman's name. The only exception would be if a character refers to someone using a title in place of a name. In Alice Walker's The Color Purple, Celie refers to her abusive husband as "Mister."
Let's tackle some far more common examples of problems making the common/proper distinction--family members.
The common nouns dad, father, mom and mother become proper when substituting for or acting like a name. My daughter doesn't call me Laurel; she calls me Mommy, Mama or Mom.
Hobbit Girl might say, "Mommy, I think you are the coolest mom ever."
In the first instance, she is addressing me "by name," that is, her name for me. In the second instance, she is talking about the role of mother, a common noun.
Here are some other examples:
"Dad!" Betsy called. "Where are you, Daddy?"
She turned to Hazel with a knowing smile. "My daddy can fix anything, just you wait."
"Aw, hogwash," Hazel said. "All your daddy can fix are martinis."
"I'm gonna tell Dad what you said. He'll whup you good, Hazel Dawkins."
Extended family such as aunts and uncles often have these titles appended to names in a fashion similar to sir.
For example:
Aunt Jo was the nicest sort of aunt. A cushiony couch of a woman, Auntie kept her hearth fire burning and all her candy jars full. Liesl wished she could live with her aunt forever and ever. She'd stop calling her Aunt Jo and start calling her Mama.
Hope that helps clarify things for you!
What capitalization conundrums trip you up most?