Either

Either is an English pronoun, adjective, conjunction and adverb. As a pronoun or adjective it means one, or the other, of two choices. As an adjective, it can also mean both of two possibilities. As a conjunction, it means one of two or more choices. As an adverb, it means "likewise" or "also", and is used for emphasis after a negative statement. Its origin is from Old English ǽghweþer, which literally analyses as a compound word "any - whether." In some constructions it may also mean both of the two choices. (ie The hallway was lined on either side with shelving.)

Either/or means "one or the other." It usage versus the simple or structure is often for emphatic purposes, sometimes intending to emphasize that only one option is possible, or to emphasize if there are only two options. Its use in a sentence lets the reader/listener know in advance that a list of two or more possibilities will be given. Its negative is neither/nor, meaning "none of them".

Pronunciation

Either has two main different pronunciations in modern English. The pronunciation [ˈiːðər] EE-dhər prevails in American English. The pronunciation [ˈaɪðər] ee-dhər is associated with British English and Canadian English, but it is not universal in either place, nor in Australian English or other dialects that take their lead from British English.

A recurring urban legend says that the ee-ther pronunciation originated with King George I or another of the Hanoverian kings of Great Britain; the king was a German who did not speak English as a native language, and was misled by English spelling. The new royal pronunciation was imitated by his courtiers, and as such became a new form. It is not likely that this is the source of the ee-ther pronunciation - before English spelling was fixed, it tended to be phonetic; as early as the 13th century, there are examples of the first vowels being spelled "ai", which would correspond to ee-ther.

An Ira Gershwin song, Let's Call the Whole Thing Off, opens with the words "You say ee-ther and I say ei-ther", and concerns a couple who lament the strain put on their relationship by pronunciation differences (and the different social backgrounds which they imply). In the end, happily, love conquers phonetics.

The 'ee-ther' pronunciation forms with the word 'either' one of the few minimal pairs demonstrating that the difference between the voiced dental fricative and the unvoiced dental fricative is phonemic in English.

Usage

A frequent difficulty in English usage concerns the permissibility of using either to refer to more than one alternative. Generally, either refers to exactly two alternatives. When there are more, linguistic prescriptivists counsel the use of any. One generally accepted exception to this rule is when either is used as a conjunction to introduce a string of either/or alternatives: Either she will sink, swim, or get out of the pool.

Any is not used as a conjunction, and cannot be used in this context.

As an adjective, either can mean both. For example, a driveway that is lined by palm trees "on either side" means that the driveway is lined with palm trees on both sides.

In the context of a legal contract, the term "either" can refer to several parties and/or beneficiaries. Real estate contracts often use "either" to include the buyers, sellers, and their agents.

Logic, law, and philosophy

Either and or are occasionally misleading terms in the sometimes loose interface between English and logic. They can be used to mean a simple logical disjunction between two alternatives (either one, or the other, or both); but either...or frequently implies an exclusive disjunction between two incompatible alternatives.

The inherent ambiguity in either and or is occasionally of import in law, such as in the interpretation of statutes and contracts. In the law of investments, an "either/or" order is an order given to a stockbroker, for which the execution of one automatically cancels the other; this is typically done to combine a "buy limit" order, which will be executed if the price is below a certain point, and a "buy stop" order, executed if the price is above that point. However, in mathematics, either/or is commonly used allowing the possibility of both options to hold true.

In philosophy, the first book Søren Kierkegaard published under a pseudonym was titled Either/Or (Danish: Enten/Eller). Written under the name Victor Eremita (Latin: the Victorious Hermit), the book contains his reflections on aesthetics and ethics, and argued against the Hegelian dialectics of thesis, antithesis, and synthesis; Kierkegaard concludes that neither aesthetics nor ethics offer a way out of the human race's existential despair, and concludes that only a leap of faith can solve that problem, arguing that making such a leap cannot have, and does not need, a rational justification*.