These are the rules of capitalization. The general rule is pretty simple. Do not use a capital letter unless it is absolutely required.
There are certain capitalization conventions that are used throughout this document. Those conventions are defined here:
When this document states to capitalize something without further definition, we mean that each word in the string should begin with a capital letter.
When capitalize is followed by a further descriptor, this descriptor enunciates which words should be capitalized and which shouldn’t.
Capitalize the first word
Lowercase the if it precedes a title then capitalize the title
Capitalize dates but lowercase prepositions in them
Fourth of July
Title case is the most common form of title and headline capitalization and is found in all four major title capitalization styles (Chicago style, APA style, MLA style, and AP style). Title case is also commonly used for book titles, movie titles, song names, plays, honorifics, etc. In general, the following capitalization rules apply across the four styles in title case:
Capitalize the first word in the title
Capitalize the last word in the title
Capitalize the important words in the title
Important words in that last bullet generally refer to:
Adjectives (tiny, large, etc.)
Adverbs (quietly, smoothly, etc.)
Nouns (tablet, kitchen, book)
Pronouns (they, she, he)
Subordinating conjunctions (when fewer than 5 letters)
Verbs (write, type, create)
While the above words are generally capitalized in titles regardless of style, there are some words that are generally not capitalized when using title case.
Articles (a, an, the)
Coordinating Conjunctions (and, but, for)
Short (fewer than 4 letters)
Prepositions (at, by, to, etc.)
The other major type of title capitalization standard is sentence case. Sentence case simply means you capitalize the first letter of a sentence, proper nouns, and nothing else as opposed to capitalizing almost every first letter in title case. It is the same across all four styles.