I recently edited a memoir in which the narrator’s parents were among the cast of characters. The book was written from a child’s perspective, and Mom and/or Dad — central figures in a child’s life — made appearances on nearly every page. I also made changes to the capitalization of “mom” and “dad” on nearly every page. Sometimes “Mom” was capitalized and “dad” was lowercased, and vice versa.
Shortly after I submitted the edits, the client emailed to ask why the capitalization is inconsistent. She didn’t doubt my edits — rather, she wanted to understand why a single word can behave in different ways. It’s a great question, and it’s also a common question. The answer is that proper nouns are capitalized and common nouns aren’t. In other words, when “Mom” and “Dad” are used in place of a person’s name, they should be capitalized. When “mom” and “dad” describe a generic parental relationship, they’re lowercase. Here’s an example:
After school, Mom took me to soccer practice.
After school, my mom took me to soccer practice.
These sentences are nearly identical. The difference is that “Mom” functions as the parent’s name in the first one whereas “mom” functions as a generic word for “parent” in the second one. You can figure out whether to capitalize by replacing “mom” with her name. If the sentence works with the name inserted, capitalize. If it sounds funny, don’t capitalize.
After school, Jane took me to soccer practice.
The sentence sounds natural, so capitalize “Mom” since it functions as a person’s name.
After school, my Jane took me to soccer practice.
The sentence sounds strange, so make “mom” lowercase since it describes a generic parental relationship.
The rule is the same one we apply when we write “my school” (common noun) vs. “Howe Elementary School” (proper noun) or “my teacher” (common) vs. “Mr. Stevenoski” (proper), or “my parents” (common) vs. “Larry and Susan” (proper). For most writers, choosing whether to capitalize is easy in all of these instances; we’d never consider capitalizing the common noun because it’s a generic descriptor, not a specific name. Similarly, we wouldn’t consider lowercasing the names since names are always capitalized.
Most of us intuitively understand these rules. It just seems foggier with mom/Mom because we use the same word for both the name and the generic descriptor. However, when we examine how the word functions within the sentence, capitalization becomes clear.