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 these words were capitalized, and sometimes they were lowercased.
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. Luckily, the answer is simple. 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’re capitalized. When “mom” and “dad” describe a generic parental relationship, they’re lowercased. Here’s an example:
After school, Mom took me to my piano lesson.
After school, my mom took me to my piano lesson.
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 “Mom” — just like you would if you were using her actual name.
After school, Susan took me to my piano lesson.
The sentence sounds natural, so “Mom” is capitalized since it functions as a person’s name.
After school, my Susan took me to my piano lesson.
The sentence sounds strange, so “mom” needs to be lowercased since it’s not functioning as a person’s name.
This is the same rule we apply when we write “my school” (common noun) vs. “Lincoln High School” (proper noun) or “my teacher” (common) vs. “Mr. Stevenoski” (proper), or “my parents” (common) vs. “Susan and Larry” (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 not a name. Similarly, we wouldn’t consider lowercasing the names since we know that names are always capitalized.
Most of us intuitively understand the rule for when to capitalize a noun. 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.