General principles for the possessive apostrophe; Source: Wikipedia
Summary of rules for most situations
Possessive personal pronouns, serving as either noun-equivalents or adjective-equivalents, do not use an apostrophe, even when they end in s. The complete list of those ending in the letter s or the corresponding sound /s/ or /z/ but not taking an apostrophe is ours, yours, his, hers, its, theirs, and whose.
Other pronouns, singular nouns not ending in s, and plural nouns not ending in s all take ’s in the possessive: e.g., someone’s, a cat’s toys, women’s.
Plural nouns already ending in s take only an apostrophe after the pre-existing s when the possessive is formed: e.g., three cats’ toys.
Basic rule (singular nouns)
For most singular nouns the ending ‘s is added; e.g., the cat’s whiskers.
If a singular noun ends with an s-sound (spelt with -s, -se, for example), practice varies as to whether to add ‘s or the apostrophe alone. A widely accepted practice is to follow whichever spoken form is judged better: the boss’s shoes, Mrs Jones’ hat (or Mrs Jones’s hat, if that spoken form is preferred). In many cases, both spoken and written forms differ between writers. (See details below.)
Basic rule (plural nouns)
When the noun is a normal plural, with an added s, no extra s is added in the possessive; so the neighbours’ garden (where there is more than one neighbour) is correct rather than the neighbours’s garden.
If the plural is not one that is formed by adding s, an s is added for the possessive, after the apostrophe: children’s hats, women’s hairdresser, some people’s eyes (but compare some peoples’ recent emergence into nationhood, where peoples is meant as the plural of the singular people). These principles are universally accepted.
A few English nouns have plurals that are not spelled with a final s but nevertheless end in an /s/ or a /z/ sound: mice (plural of mouse; also in compounds like dormouse, titmouse), dice (when used as the plural of die), pence (a plural of penny, with compounds like sixpence that now tend to be taken as singulars). In the absence of specific exceptional treatment in style guides, the possessives of these plurals are formed by adding an apostrophe and an s in the standard way: seven titmice’s tails were found, the dice’s last fall was a seven, his few pence’s value was not enough to buy bread. These would often be rephrased, where possible: the last fall of the dice was a seven.[note 1]
Basic rule (compound nouns)
Compound nouns have their singular possessives formed with an apostrophe and an added s, in accordance with the rules given above: the Attorney-General’s husband; the Lord Warden of the Cinque Ports’ prerogative; this Minister for Justice’s intervention; her father-in-law’s new wife.
In such examples, the plurals are formed with an s that does not occur at the end: e.g., attorneys-general. A problem therefore arises with the possessive plurals of these compounds. Sources that rule on the matter appear to favour the following forms, in which there is both an s added to form the plural, and a separate s added for the possessive: the attorneys-general’s husbands; successive Ministers for Justice’s interventions; their fathers-in-law’s new wives. Because these constructions stretch the resources of punctuation beyond comfort, in practice they are normally reworded: interventions by successive Ministers for Justice