The more you get right in your manuscript before submitting to an editor, the less work they'll have to do on mechanical issues ---- and the more time they can spend on making your writing shine. Plus, investing the time to learn the conventions of fiction style* can save you money.
*This is Articulate Editing house style, which is based on generally accepted fiction conventions (in the US) and a pinch of personal opinion. Most of what is written here will apply across the board, but if a guideline has wiggle room or exceptions, I'll let you know—and if you're working with a publisher, make sure you follow their specifications as their house style may differ slightly. (This style guide also applies to creative nonfiction.)
Many people think that capitalization is part of grammar, but it's actually a matter of convention and style. While certain types of words are always capitalized, others depend on usage conventions that are flexible or change over time (usually from capital to lowercase).
While practically no English-speaking adult needs to be told to capitalize a person's name or the first word in a sentence, when to capitalize a military or political title can be confusing, partly because the convention can vary depending on whether you're writing a newspaper article, a book, or a government document.
Since we're only concerned with fiction style here, don't worry when you see different treatments elsewhere. That said, let's get on with it, shall we?
A proper noun is a noun/name that designates a particular thing, not one of a category of things. Most proper nouns are capitalized, as are some terms that are derived from proper nouns. Convention changes over time, and sometimes usage is in flux: Internet is still capitalized in Merriam Webster's Unabridged, but both AP and Chicago downgraded it to lowercase a few years ago.
Terms derived from proper nouns are often capitalized. (Except when used with a nonliteral meaning or when the connection to the proper noun has become remote.) A dictionary will be helpful here. Most fiction editors use Merriam Webster's Collegiate.
pasteurize (from Louis Pasteur, the man who sanitized milk)
bowdlerize (from Thomas Bowdler, the man who sanitized Shakespeare)
Adjectives derived from proper names are normally capitalized, unless the connection to the proper name has become remote over time.
herculean effort (from Hercules, Greek hero)
quixotic quest (from Don Quixote by Cervantes)
Capitalize names with lowercase particles when they begin a sentence.
"Von Trapp, did you invite Miss duBois?"
"DuBois, did you RSVP to Mr. von Trapp?
Capitalize epithets and nicknames used instead of (or as part of) a person's name.
Alexander the Great
But ... If the nickname is given in addition to the actual name, it's surrounded by quotation marks.
Dwayne "the Rock" Johnson (don't capitalize "the" in a nickname)
Dwayne Johnson, "the Rock," is both a wrestler and an actor. (No need for parentheses.)
Terms of endearment and insults are generally lowercased.
"Where shall we eat tonight, darling?"
"Hey, jerkface, cut that out!"
But ... if elevated to nickname status, they can be capitalized. (A wife wouldn't ask, "Where's Darling?" when her husband is missing, but a kid might dub his constant tormentor Jerkface, and use it as if it were his name.)
"Oh no! It's Jerkface! Let's get out of here."
If an insult or endearment contains a title such as Mister, Madame, or Doctor, it's always capitalized, even if it's a one--off rather than an established nickname.
"Hey, Mr. Smarty Pants, where do you think you're going?"
"Hey, mister, where you going?"
Kinship names are lowercased unless being used in place of a person's name.
I love you, Grandma.
I love my grandmother.
I have to go visit Aunt Ruth.
I have to go visit my aunt Ruth.
Deliberately lowercased names should be respected. This is true for personal names and brand names both—but avoid starting a sentence with them.
Titles and honorifics
Titles (whether military, political, professional, religious, or academic) follow the same general rule. They're capitalized when they precede a given name and are used as part of that name, but are lowercased when used generally.
Barack Obama, president of the United States
The president is giving his inaugural address today.
John Donaldson, professor of literature
Lowercase the title when used as an noun modifier rather than as part of the name.
the former president Donald Trump
Capitalize the title when used as a direct address.
It's lovely to meet you, Mr. President.
Sergeant, what are your orders, sir? ("sir" is lowercased; see below)
Please pray for me, Reverend, for I've lost my way.
Lowercase generic titles that describe an occupation and are used as noun modifiers.
the architect Frank Gehry
Capitalize honorifics and terms of respect.
the First Lady
Your Royal Highness
Sir Ian McKellen
my lady, my lord
National or ethnic group names are capitalized, as are adjectives based on those names.
an Irish person
the Sioux tribe
Skin color: Up until recently, the advice was to lowercase black and white, but capitalizing Black is quickly becoming the standard to shift the emphasis from skin color to shared cultural identity. And for consistency, White is often capitalized, too. The New York Times published an article about their decision to capitalize Black last year.
The trend is to not hyphenate words that indicate dual heritage such as Asian American. Without the hyphen, the first term modifies the second, so the person in question is an American (noun) with origins in Asia (adjective). With the hyphen, the implication is that the person in question isn't fully American, but some kind of hybrid. For more, check out this article by Henry Fuhrmann at Conscious Style Guide.
Categories of class, sexual orientation, or ability are lowercased.
the middle class
Organizations, government entities, institutions, religious groups, and corporations (including brand names) are usually capitalized. The generic word is usually lowercased.
The Federal Bureau of Investigation
the Albany City Council
the city council
But unofficial political groups are usually lowercased.
Systems of thought are lowercased even though adherents to an official group based on that system of thought are capitalized.
the Communist party
I suspect she subscribes to the tenets of communism.
She's a Communist.
Certain government entities are lowercased.
executive branch (as well as judicial branch and legislative branch)
Names of deities are capitalized, no matter the religion.
"Nicknames" of deities and revered persons are capitalized.
the Blessed Virgin
Pronouns referring to deities are generally lowercased unless the publisher or author prefers reverential capitalization. (Most Christian presses lowercase pronouns referring to God, and so do the two most popular versions of the Bible.)
"Church" is generally lowercased unless part of the name of a specific church. (Unless the author or publisher prefers otherwise.)
First Baptist Church
the early church
The names of spiritual places are normally lowercased. (Unless the author or publisher prefers otherwise.)
The names of scriptures are capitalized. (And not italicized.)
the Holy Bible
New International Version
Significant religious concepts and events are often capitalized.
the Second Coming
Doctrines are usually lowercased.
The names of places (including rivers, mountains, streets, regions, buildings, monuments, official rooms) are capitalized (whether real or imaginary). The "Geographical Names" section in Merriam Webster's Collegiate Dictionary is a great resource.
Yellowstone National Park
the West Coast
the Statue of Liberty
the Oval Office
So are adjectives and nouns derived from them.
Nicknames for places are capitalized. No need for quotation marks.
the Big Apple
the Golden State
the City (what New Yorkers call Manhattan)
A generic term is lowercased unless it's part of the name.
I'm traveling down the river known as the Mississippi.
I'm traveling down the Mississippi River
Nouns that are modified by a proper noun but aren't officially part of the name are lowercased.
the California desert
Nile River valley
the Capitol building (the official name is "the U.S. Capitol)"
the Sonoran Desert
the White House
Directions are lowercased unless part of a place name or an established region.
He drove west until he reached the ocean.
a north-east wind
Though born in the Midwest, she acted like a typical Southern belle.
the North Pole
Time periods and cultural movements
Numerically expressed time periods are usually lowercased.
the twentieth century
the eighteen hundreds
the Roaring Twenties
Historical periods and cultural movements are sometimes capitalized and sometimes lowercased. This depends on tradition, whether or not the name is derived from a proper name, and whether there's potential for confusion with a general term (romantic period vs. the Romantic period). When in doubt, look it up.
the postmodern era
the early medieval period
the Middle Ages
Edwardian (named after king Edward VII of England)
the Victorian era (named after Queen Victoria)
Pre-Raphaelite (named after the artist Raphael)
Historical events are usually capitalized, but not always. The dictionary is your friend.
the Cold War
the Battle of the Bulge
the gold rush
the women's liberation movement
Prehistoric time periods are capitalized even though modern time periods are often
the Bronze Age
the Stone Age
the information age
Geological time periods are capitalized, but the following generic term is lowercased.
the Proterozoic eon
the Triassic period
Days of the week and months of the year are capitalized, but seasons are lowercased.
spring, summer, fall, winter
Time zones are lowercased when spelled out.
eastern standard time (EST)
Academic titles follow the general rule for titles above.
Names of academic subjects are lowercased unless derived from a proper noun or part of a department name or course title.
He double-majored in philosophy and biology.
I took an English literature class as a freshman.
The young professor worked in the Department of Comparative Religion.
I signed up for Geology 201.
Student year designations are lowercased.
freshman, sophomore, junior, senior
Academic degrees are lowercased when used generically.
master of business analytics
bachelor of science
Capitalize brand names that are still valid trademarks. Wherever possible, substitute for a generic term.
Many formerly trademarked brand names have become legally genericized.
Throw it in the dumpster.
I need an aspirin for this headache.
Some brand names that have not become legally genericized are used as verbs. You'll see these lowercased, but it's still a good idea to respect the wishes of these companies and capitalize. Or use a generic term instead.
Will you Photoshop me out of the picture? (try "retouch" instead)
Just Google it. (try "do an internet search" instead)
I need to Xerox this document. (try "copy" instead)
Capitalization of titles
Use headline style when mentioning titles in your fiction writing.
➣ Capitalize the first and last word of the title.
➣ Capitalize all major words: nouns, pronouns, verbs, adjectives, and adverbs.
➣ Lowercase prepositions regardless of length, unless they're used as a modifier (Down Style) or as part of a phrasal verb (Screw Up).
➣ Lowercase the conjunctions and, but, for, or, and nor; capitalize all other conjunctions.
➣ Lowercase to, not only when it's a preposition, but also when it's part of an infinitive.
➣ Lowercase the articles a, an, and the.
➣ Capitalize the first word after a colon.
If there's a hyphenated word in the title ...
➣ Always capitalize first part of the compound word.
➣ Capitalize following parts unless they're coordinating conjunctions, articles, or prepositions.
➣ If the first part is a prefix, not a stand--alone word, then the second part shouldn't be capitalized (e.g., Anti-immigration Reform), unless it's a proper noun or an adjective derived from a proper noun (e.g., Pro--Trump Protestors).
➣ Capitalize the second part of spelled--out numbers and simple fractions (e.g., Thirty--Seven, Two--Thirds).
Capitalization for emphasis
Italics, not all caps, should be used to add emphasis to a word or phrase. The exceptions are books for young children and some middle grade books.
All caps can be used for shouting in dialogue (just don't overdo) and loud sounds in narrative.
And you can capitalize platonic ideals and ironic titles.
"I believe in Truth with a capital T."
"I've been a Bad Boy."
Formal oaths and pledges are usually lowercased.
the presidential oath of office
the Pledge of Allegiance
Signs are usually capitalized headline style in the text. (Though if they're long, it's better to use quotation marks and sentence style).