The use of capital letters in punctuating sentences has been discussed in Punctuation and Style pages; their use to distinguish proper nouns or 'names' from ordinary words is subject to wide variation in practice. The standard OUP style is outlined below, but the most important criterion is consistency within a single piece of writing.

1. Capital letters are used for the names of people and places (John Smith, Paris, Oxford Street, New South Wales, the Black Sea, the Iron Duke); the names of peoples and languages and derived words directly relating to them (Englishman, Austrian, French, Swahili, Americanise); the names of institutions and institutional groups (the Crown, the Government, the British Museum, the House of Representatives, the Department of Trade); the names of religious institutions and denominations and their adherents (Judaism, Nonconformism, Methodist, Protestants) and of societies and organisations (the Royal Society); the names of months and days (Tuesday, March, Easter Day); abstract qualities personified (the face of Nature, O Death!) or used as sobriquets (a Blue in university sport, a Red = communist); and names of other non-personal things (the Flying Scotsman).

Note that the Baptist Church is an institution, but the Baptist church is a building; a Democrat belongs to a political party, but a democrat simply supports democracy; Northern Ireland is a name with recognised status, but northern England is not.

2. A capital letter is used for words derived from a proper name, if the connection with the name is direct, or felt to be continuing (Christian, Homeric, Marxism), but not if it is more remote or conventional (chauvinistic, quixotic, guillotine).

3. A capital letter is used by convention in many names that are trade marks (Elastoplast, Filofax, Hoover, Xerox) or are otherwise associated with a particular manufacturer etc. (Jaguar, Spitfire). Some proprietary terms are now conventionally spelt with a lower case initial (baby buggy, biro, cellophane, jeep), and this is generally true of established verbs derived from proprietary terms (to hoover, to xerox).

4. Capital letters are used in titles of courtesy or rank, including compound titles (His Royal Highness the Prince of Wales, President Carter, Sir John Smith, Lord Chief Justice, Lieutenant-Colonel, Vice-President, Your Grace, His Excellency).

5. A capital letter is used for the personal pronoun I and for the interjection O.

6. A capital letter is used for the deity (God, Father, Allah, Almighty). However, the use of capitals in possessive determiners and possessive pronouns (in His name) is now generally considered old-fashioned.

7. Capital letters are used for the first and other important words in titles of books, newspapers, plays, films, television programmes, etc., and in headings and captions (The Merchant of Venice, Pride and Prejudice, Book of Common Prayer, New Testament, Talmud, Guide to the Use of the Dictionary).

8. Capital letters are used for historical events and periods (the Dark Ages, Early Minoan, Perpendicular, the Renaissance, the First World War); also for geological time divisions, but not for certain archaeological periods (Devonian, Palaeozoic, but neolithic).

9. Capital letters are frequently used in abbreviations, with or without full stops (BBC, DoE, M Litt).

10. A capital letter is used for a compass direction when abbreviated (N., NNE, NE) or when denoting a region (unemployment in the North).

11. A capital letter is frequently used to begin a line of English verse.

12. The use of a capital letter elsewhere than at the beginning of a word is seen in certain names (MacDonald, O'Reilly) and in some trade marks, and is conventional in some foreign languages.