In general, avoid unnecessary capitals. Use a capital letter only if you can justify it by one of the principles listed here:
Proper nouns Capitalize nouns that constitute the unique identification for a specific person, place or thing (John, Sue, Charlotte). Some words, such as the examples just given, are always proper nouns. Some common nouns receive proper noun status when they are used as the name of a particular entity (General Electric, Gulf Oil).
Proper names Capitalize common nouns such as party, river, street and west when they are an integral part of the full name for a person, place or thing (Democratic Party, Mississippi River, Fleet Street, West Virginia). Lowercase these common nouns when they stand alone in subsequent references (the party, the river, the street). Lowercase the common noun elements of names in all plural uses (the Democratic and Republican Parties, the corner of 3rd and Davidson streets, lakes Erie and Toronto).
Additional guidelines Consult the AP Stylebook for the following entries that provide additional guidelines: animals, brand names, building, committee, congress, datelines, days of the week, directions and regions, family names, food, foreign governmental bodies, foreign legislative bodies, geographic names, governmental bodies, heavenly bodies, historical periods and events, holidays and holy days, legislature, months, monuments, nationalities and races, nicknames, organizations and institutions, planets, plants, police department, religious references, seasons, trademarks and unions.
Popular names Some places and events lack officially designated proper names but have popular names that are the effective equivalent (the Combat Zone -- a section of downtown Boston, the Main Line -- a group of Philadelphia suburbs, the South Side -- of Chicago, the Badlands -- of South Dakota, and the Street -- the financial community in the Wall Street area of New York City). The principle applies also to shortened versions of the proper names of one-of-a-kind events (the Series -- for the World Series, the Derby -- for the Kentucky Derby). This practice should not, however, be interpreted as a license to ignore the general practice of lowercasing the common noun elements of a name when they stand alone.
Derivatives Capitalize words that are derives from a proper noun and still depend on it for their meaning (American, Christian, Christianity, English, French, Marxism, Shakespearean). Lowercase words that are derives from a proper noun but no longer depend on it for their meaning (french fries, herculean, manhattan cocktail, pasteurize, quixotic, venetian blind).
Sentences Capitalize the first word in a statement that stands as a sentence. In poetry, capital letters are used for the first words of some phrases that would not be capitalized in prose.
Compositions Capitalize the principle words in the names of books, movies, plays, poems, operas, songs, radio and television programs, works of art, etc.
Titles Capitalize formal titles when used immediately before a name. Lowercase formal titles when used alone or in constructions that set them off from a name by commas. Use lowercase at all times for terms that are job descriptions rather than formal titles (chairman, company spokesperson).
most words add ‘s’ (girls, ships, villages)
words ending in ch, s, sh, x and z add ‘es’ (churches, lenses, parishes, glasses, boxes, buzzes)
words ending in is change ‘is’ to ‘es’ (oases, parentheses, theses)
words ending in y if ‘y’ is preceded by a consonant or ‘qu’, change ‘y’ to ‘i’ and add ‘es’ (armies, navies, soliloquies, tries). Otherwise, add ‘s’ (keys, boys, delays)
words ending in o If ‘o’ is preceded by a consonant, most plurals require ‘es’ (buffaloes, dominoes, heroes, potatoes). But there are exceptions.
words ending in f In general, change ‘f’ to ‘v’ and add ‘es’ (leaves, shelves). Roof/roofs is an exception.
Latin endings Latin-root words ending in ‘us’ change to ‘i’ (alumnus/alumni), but there are exceptions (syllabus/syllabuses, prospectus/prospectuses). Most ending in ‘a’ change to ‘ae’ (alumna/alumnae). Formula/formulas is an exception. Most ending in ‘um add ‘s’ (memorandums, referendums, stadiums). Among those that still use the Latin ending are addenda, curricula, media.
form change man/men; child/children; foot/feet; mouse/mice, etc.
words the same in singular and plural corps, chassis, deer, moose, sheep, etc.
words plural in form, singular in meaning measles, mumps, news, headquarters, grits, scissors
compound words Those written solid add ‘s’ at the end: cupfuls, handfuls. Those that involve separate words or words linked by a hyphen, make the most significant word plural.
significant word first attorneys general, presidents-elect, daughters-in-law.
significant word in the middle assistant attorneys general, deputy chiefs of staff.
significant word last assistant professors, lieutenant colonels, deputy sheriffs.
words as words In general, never use an apostrophe to pluralize a word: No ifs, ands or buts
Most ending in ‘es’, ‘s’ or ‘z’ add ‘es’: Charleses, Carneses, Joneses, Gonzalezes
Most ending in ‘y’ add ‘s’, even if preceded by a consonant: the Duffys, the Kennedys, the two Kansas Citys. Exceptions include Alleghenies and Rockies.
For others, add ‘s’: the Carters, the McCoys, the Mondales
figures/numbers Add ‘s’ (no apostrophe): The temperatures will drop in to the 20s. The only shoes left where size 7s.
decades (abbreviated) To abbreviate, put an apostrophe at the beginning and add ‘s’: My grandparents grew up in the 1930s. I love ‘80s music.
single letters The only case in which apostrophes are used to make something plural: Mind your p’s and q’s. School should emphasize the three R’s. If you study hard, you can make all A’s.
multiple letters Add ‘s’, no apostrophes: She burned six DVDs for me. I uploaded 12 PDFs to the website.
Generally, do not hyphenate when using a prefix with a word starting with a consonant.
Except for the words cooperate and coordinate, use a hyphen if the prefix ends in a vowel and the word that follows begins with the same vowel (pre-existing).
Use a hyphen if the word that follows is capitalized.
Use a hyphen to join doubled prefixes like sub-subparagraph.
Common prefixes include the following: a- or an-, ante-, anti-, auto-, by-, circum-, co-, com- or con-, contra-, de-, dis-, en-, ex-, extra-, hetero-, homo-, hyper-, il-, im-, in-, ir-, inter-, intra-, macro-, micro-, mid-, mini-, mono-, non-, omni-, post-, pre-, pro-, sub-, super-, syn-, trans-, tri-, un- and uni-.