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Some style guides consider contractions to be informal, and therefore would not be used in certain contexts. Beyond that, there's no grammatical restraint, either on they are vs. they're or the r...
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#4: Attribution notice removed
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/22001 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Some style guides consider contractions to be informal, and therefore would not be used in certain contexts. Beyond that, there's no grammatical restraint, either on _they are_ vs. _they're_ or the referent of the pronoun _they. They_ is the plural pronoun for both _he/she_ and _it._ So "They are in the back room" can refer to two or more people _or_ two or more objects. (I would not, however, mix them. If you have a woman in the back room who is holding a bunch of printed forms, you would say "She has them in the back room.")