The English language has two character sets for the standard 26 letters that make up the alphabet. Uppercase letters (AKA capital letters) look like this:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
- Proper nouns should always have a capital the initial letter (Tem42, America). So should proper adjectives (Victorian, Eastern)
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The first word in a sentence should be initiated with a capital letter.
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The first letters of each word in a title (of a book, paper, movie, song, etc.), but excluding 'unimportant words' (and, the ,of, a, etc.), unless they are the first word in the title. (A Night to Remember, The Old Man and the Sea.) The exact rules regarding which words are uncapitalized are flexible, but generally, all articles, prepositions and conjunctions are kept lowercase.
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The first letter of a person's honorific or title (Mr., Doctor, Lord, His Royal Highness).
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Acronyms (GNU, SCUBA) and initialisms (OMG, LOL, TNT) should be all caps.
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The word 'I' should always be capitalized, if you wish to be grammatically correct. If you are using the word 'O' (O, mighty king) it too should be upper case.
- You may also find them used for EMPHASIS, which may or may not be A Good Thing.
- Words referring to the JCI God should have the first letter in uppercase (His will, He). Zeus is capitalized, but his will is not (usually).
Long, long ago (in the 1700s and before, to be exact), all nouns were capitalized; you can see this in the works of Benjamin Franklin and Jonathan Swift, among others.