When to Apostrophise

Singular Possessives are Formed With 's

...even if the name ends with an "s".

The only exceptions are: historial names ending in -is and -es, such as Moses and Isis, and Jesus. Hence: Moses', Isis', Jesus'. Note that the normal way of creating the possessive with Jesus is becoming popular, hence Jesus's, and that prepositional phrases are advised with ancient names: words of Moses, knowledge of Isis.

Plurals are Never Formed with 's

Examples:

WRONG: There were several bridge's along the river.
RIGHT: There were several bridges along the river.

The only exception is with the plurals of things which would be pronounced incorrectly if formed with a plural in the usual manner, usually single letters and very short words. For example: "there are three s's in this sentence". Acronyms' plurals may also be formed with an 's, for example "URI's", but this is stylistically unpreferred.

Possessive Pronouns

Possessive pronouns never, ever have apostrophes in them. Never. Don't do it.

WRONG: The dog chased it's tail.
RIGHT: The dog chased its tail.
WRONG: Suzi thought that Mary's cakes were comparable with her's.
RIGHT: Suzi thought that Mary's cakes were comparable with hers.

It's counter-intuitive because you're taught to form possessives of nouns using 's—why should pronouns be any different? But "it's" means "it is". The easy way to tell whether your use is correct is to remember that "it's" always expands to "it is", and test whether that sounds right. So "the dog chased it's tail" becomes "the dog chased it is tail", which is clearly incorrect. Same with you're and your: you're is a contraction of "you are", so "it is you're turn" expands to "it is you are turn", which is wrong.

Plurals' Apostrophes are Formed With an ' Only

Example: the heading.

WRONG: All the student's rights.
RIGHT: All the students' rights.

Exception: when the plural doesn't end with an s, then 's is used. For example: it was the sheep's fault, or it was the mice's fault.

Corner Cases

The first is: US, as in the United States. If I want to say, "the economy of the US", is that "the US's economy", or "the US' economy"? Technically, it should be the latter because US is a plural: more than one state. But... it seems very silly. Moreover, the US seems like a single entity; I say: "the US is an odd place", not "the US are an odd place". Roy Fielding uses the acronym "URI" as its own plural occasionally, which is confusing.

The second problem is where company names are formed with an apostrophe-s. For example, Lloyd's. Therefore, the possessive of Lloyd's should be Lloyd's's... which is stylistically horrible. The general advice on the Lloyd's problem is to formulate it preposionally: "the advice of Lloyd's".

Sean B. Palmer