This little label is added to a word to denote that something belongs to someone or something, provided the possessor is singular.
Example: the accountant's books, the company's marketing strategy Note that the amount of things possessed does not influence this rule whatsoever, i.e. there are several books, but only one strategy. Only the person or thing to which these belong is important, i.e. the one accountant and the one company.

As you may have guessed, the change in place of the apostrophe means we are now dealing with plural possessors.
Example: the employers' association Here, the apostrophe added to the end of the plural possessor indicates that the association belongs to several employers.

However, as we know, there are some irregular plurals in English. For these, we revert to using the initial 's label, as these words do not themselves end naturally in an s.
Example: women's magazine, children's toys
This denotes that the items belong to the women and the children respectively.

For singular nouns already ending in an s, as is often the case with people's names, there is some grey area. Both James's wife and James' wife are correct. According to the rule, as James is only one person, we would use the former. The latter, however, is considered to be simpler and clearer.

This frequently used example brings both functions of the apostrophe into play. The possessive pronouns mine, yours, hers, his, ours, theirs, whose and its never take an apostrophe. Therefore we write, "the dog had found its bone". When written with an apostrophe, it's is the contracted form of it is. Whenever tempted to use the apostrophe, be sure you do want to say it is, which is evidently not the case here.

These are the essential rules for the use of the apostrophe in possessive cases. Some other general tips will help in areas of confusion.

a) Always use the apostrophe when talking about time periods, such as in "one month's holiday" or "four hours' delay" as they serve a possessive function. Consider that the holiday belongs to the month and the delay belongs to the hours, however absurd this may seem!

b) The apostrophe must also be applied in cases such as "the text must go to the printer's" or "our rates are lower than other companies'". This is because the implied sense is "the printer's firm" and "other companies' rates".

c) For phrases and joint ownership, put the apostrophe at the end.
Example: "the parent company's management", or "Helen and Brian's new products" provided, of course, the thing possessed belongs to both. Note that "Helen's and Brian's new products" means that both people have made separate new discoveries!

Contractions
The apostrophe's other main use is to replace letters omitted from a word that has been shortened or contracted. An example is my own surname, O'Donnell, which originally meant son of Donnell, but an apostrophe now replaces the letter f. Similarly, as we know from our last article, it's is not a possessive form, rather the apostrophe replaces the letter i to form the contracted form of it is.

The apostrophe's use in contractions falls into one of several categories:

Never forget that although contractions are widespread in spoken English and even make the speaker sound much more natural, they should always be avoided in formal writing - only in informal correspondence are they really acceptable.