Whether you capitalize "mom" depends on how you are using the word. Is it a nickname, a common noun, or a term of endearment?
Capitalization Wasn’t Always Like It Is Today
Capitalization styles shifted so much and so quickly that by the 1830s, writers were favoring lowercase letters, not only for common nouns but also for many things we would capitalize today. The reasons for the change are mysterious. The prominent usage writer H.L. Mencken speculated that the change was “probably as a result of French influence.” During this time, writers sometimes kept days of the week and courtesy titles lowercase. For example, they may have written about meeting “mr. Mencken on wednesday.”
And then I found snippet in a later book from 1894 called the “Dictionary of Printing and Bookmaking,” in which the author, Wesley Washington Pasco, bemoaned that librarians had become “wearied with the difficulty of applying an exact rule to every case,” And were insisting that the only words that should be capitalized were words at the beginning of sentences and words that were “strictly proper nouns.”
These were times of great change. Today, of course, we capitalize “Mr.” and “Wednesday” and keep words such as “power,” “age,” and “years” lowercase (all three of which are capitalized in the Constitution).
Current Basic Capitalization Rules
The most basic modern capitalization rule is to capitalize proper nouns and keep common nouns lowercase. A proper noun is the name of a person or thing, whereas a common noun is a generic descriptor. Therefore, we write about the Golden Gate Bridge (with “bridge” capitalized) and the orange bridge that crosses the bay (with “bridge” lowercase).
Names, Nicknames, and Terms of Endearment
It quickly gets complicated though. For example, we know that we capitalize someone’s name (for example, Juliette), but what about a nickname? Nicknames are capitalized, so if you always call Juliette “Northie” because she’s from Alaska, you’d capitalize “Northie” the same way you’d capitalize “Juliette.”
- Hey, Juliette, do you want to go to the movies tonight?
- Hey, Northie, do you want to go to the movies tonight?
On the other hand, you may call your husband “honey,” but you don’t capitalize “honey” the same way you’d capitalize a nickname. It’s considered a term of endearment, and those aren’t capitalized. The difference can be subtle. One trick for telling the difference between a nickname and a term of endearment is to test whether you’d use the term when talking to someone else. If you were talking to your visiting sister about your husband, would you say, “Honey called and said he’ll pick up dinner on the way home”? If not, “honey” is a term of endearment and you don’t capitalize it. (I have met people who are called “Honey” as a nickname, so it’s not a universal rule that you don’t capitalize “honey.”)