We all know that a noun is a person, a place, or a thing. A proper noun is a subset, referring to specific people, places, or things. This specificity leads to the key qualifiers of a proper noun: that it refers to something unique, or one-of-a-kind, and is therefore capitalized.
For example, the Taj Mahal is a place (and arguably a thing), but as it is the only one in the world, it’s capitalized, making it a proper noun.
You might be thinking … hang on a second. There’s more than one Sandra or Jack in the world. And that may be true, but there is only one Sandra (if that happens to be your name) who is inarguably you. So again, caps, and again, a proper noun.
Let's dig into this further.
Non-proper nouns are called common nouns and do not reference one specific or unique thing. For example, “I feel like eating a cookie.” This sentence features a common noun, cookie. Now if you said, “I feel like eating an Oreo,” this sentence has a proper noun, Oreo, because it is a unique, one-of-a-kind classification within the category of cookie. Even if you got more specific and said oatmeal cookie, there are a million and one oatmeal cookies and recipes out there, so it is another common noun (or an adjective plus a common noun, if you want to get picky).
Almost always, yes — that’s part of what makes a proper noun proper. It’s worth noting, though, that some words can function as both common and proper nouns, depending on the context. For example, “We went swimming with dolphins on our vacation,” versus, “I hope the Dolphins win this weekend.” Here we’re talking about a common noun (dolphins as a species) versus a proper noun (the Miami Dolphins football team).
If you are pairing a title with someone’s name, you will generally capitalize it. You would capitalize President Bush as a proper noun, but “The president of Walmart will be visiting the warehouse next week,” would use a lowercase title. Same goes for, “The doctor will see you now,” versus, “Doctor Banner gave me a prescription to fill.”
An eponym refers to a noun named after someone, or derived from a proper noun. Some eponyms are capitalized, some are not. Again, the difference between caps and non-caps comes down to being unique or specific. The word sandwich is an eponym named after the fourth Earl of Sandwich in the mid-18th century. While it was named for him, the noun sandwich is so broadly used now, it’s no longer capitalized.
Conversely, Marxism is a school of thought named after German political theorist Karl Marx. Because this body of teachings specifically relates to one person (Marx) and no one else, it’s capitalized.
As with most of the English language, there are weird rules out there when it comes to proper nouns. For example, we capitalize the days of the week and months of the year, but we do not capitalize the seasons.
Looking to the heavens, all of our celestial bodies are capitalized, while sun and moon generally are not, because there's more than one sun and moon in the universe. But if you are referring to a specific sun, you can capitalize it, as in, “The Sun is the star at the center of our solar system.” If that one trips you up, you could also call them Sol and Luna, our sun and moon’s alternate and truly proper nouns.
Earth is another one of those names that functions as both a common and proper noun, as in, “She dug into the earth.” As well as, “The Earth is the third planet from the Sun.”
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