A common noun is the generic name for a person, place, or thing in a class or group. Unlike proper nouns, a common noun is not capitalized unless it either begins a sentence or appears in a title. Common nouns can be concrete (perceptible to the senses), abstract (involving general ideas or qualities), or collective (referring to a group or collection).
If you want to be my best friend, the Blue Moon Diamond would be a great gift idea.
Usually, it will be quite obvious if a specific person, place, or thing is being named.
A cat may scratch you if it is teased too much.
Seriously, you should watch yourself around Fluffy.
Ray likes to hike in the mountains.
Ray has climbed Mount Everest twice.
A girl rang the doorbell this afternoon while you were out.
Gigi rang the doorbell this afternoon while you were out.
Distinguishing between common nouns and proper nouns seems easy, so why do we really need to know the difference between them? The answer to that question is this: to assign capital letters correctly.
Common Nouns Are Lowercased
A frequent spelling error people make is to capitalize common nouns unnecessarily. Some words, like president, seem to beg for a capital letter because instinctively we want to emphasize their importance. But even this lofty title is a common noun if it does not name something or someone specific (in this case, a specific president).
In the penultimate example, the fact that George Washington is named at the beginning of the sentence may tempt you to capitalize president afterward, but you must resist this temptation. Here, George Washington is referred to as the first president of the United States—that is, the first in a number of presidents of the United States. Thus, in that example, president is a common noun.
In business writing, the common compound noun board of directors is often incorrectly capitalized in this way. Although you may idolize your company’s board of directors and put them on the proverbial pedestal, board of directors is not capitalized unless it is part of a proper noun.
This principle applies to every common noun, no matter how monumental its significance. Even if your noun represents the most momentous event the universe has seen, it must be named specifically to be proper and to don its capitals.