Singular they

Singular they is the standard English gender neutral pronoun for any individual whose gender is unknown or unspecified. For this reason, "they" pronouns are one of the three most popular pronouns used for real nonbinary people, as shown in surveys (see below).

Grammatical concerns
Grammarians question and debate the validity of also using this set as "singular they," a gender-neutral pronoun for an individual whose sex isn't specified, because they feel unsatisfied by how it still works grammatically like a plural. (Similar to how we say "you are," even when referring to a singular "you.") However, "singular they" has been common usage in English for over a thousand years, as attested by many written documents. It was used so by Shakespeare, Jane Austen, the Oxford English Dictionary, Louis Carroll, C. S. Lewis, and others. Grammaticians only recently decided that it was ungrammatical to use in the singular, because it is still used with verbs as though it was a plural. Though the grammar may be questionable, it is still standard use today.

Singular "they" gets a surprising number of complaints, considering people use it all the time for people whose gender is unknown. However, there are a lot of arguments that it's correct. This article on Motivated Grammar goes through a few reasons, including the fact that it's consistently been in use since Chaucer's time, around 1400. Most people are familiar with how singular "they" works, as plenty of people use it all the time without really thinking about it.

A most comprehensive article about the history of singular "they" is Henry Churchyard's web-page, Singular 'Their' in Jane Austen and Elsewhere.

Forms
The forms of "singular they" are they, them, their, theirs, themselves (themself, theirself, theirselves).


 * Nominative: When I tell someone a joke they laugh.
 * Accusative: When I greet a friend I hug them.
 * Pronominal possessive: When someone does not get a haircut, their hair grows long.
 * Predicative possessive: If I need a phone, my friend lets me borrow theirs.
 * Reflexive: Each child feeds themself. Or: each child feeds theirself. Or: each child feeds theirselves. Or: each child feeds themselves. (See below for information on how this form's standard use varies by dialect.)

On Pronoun Island: http://pronoun.is/they On Pronouny: http://pronouny.xyz/pronouns/593c1223975dfa1000d4ec86

Reflexive form variants
There are several versions of the reflexive form of this pronoun: "themself," "theirself," "theirselves," and "themselves."

Because both "themself" and "theirself" are for talking about a single person, they're both considered non-standard or informal usage, despite the hundreds of years of common usage. The plural form of "themselves" is supposed to be more formal, but can sound strange when used for a single person, because they are not several "selves," but one "self". This is another part of the plural/singular "they" problem.
 * Themself. The Oxford Dictionary says that "themself" has been used since the 14th century for a person of unknown sex.
 * Theirself, theirselves. Dictionary.com says that "theirself" has also been used for this since about 1300. The Free Dictionary adds that "theirself" and "theirselves" are more common in southern and midland US English.

People differ about which version of the reflexive form they prefer. If someone asks to be called by "they" pronouns, it might be a good idea to ask which form of the reflexive form they prefer.

Singular they in use in fiction for nonbinary characters

 * In a short sci-fi story by Benjanun Sriduangkaew, "Silent Bridge, Pale Cascade" (2013), one of the characters is described as a "neutrois," and called by "they" pronouns.
 * In Kameron Hurley's fantasy novel, Empire Ascendant, all people in a consent culture get to choose which of the five gender roles they identify with. Hurley calls characters who are "ungendered" by singular they pronouns.
 * In K. A. Cook's anthology Crooked Words, the character Chris asks to be called by "they" pronouns. Chris is in the short stories "Blue Paint, Chocolate and Other Similes" and "Everything In A Name."

Singular they in use for real nonbinary people

 * Some notable nonbinary people who ask to be called by "singular they" pronouns include writer Ivan E. Coyote, actor Tom Phelan, actor Jiz Lee, singer-songwriter Rae Spoon, and rapper Raeen Roes.
 * Deborah Rogers mentions having a trans male student who asked to be called by "they" pronouns.
 * "Singular they" has been the most popular pronoun in the Gender Census, at 74% acceptance in 2015, rising to 77% in 2016, and rising again to 80% in 2017. In 2018, the pronoun fell down to 77.4%.