I have never been a fan of the explanation that apostrophes are used to show ‘possession’; I prefer to use ‘association’. After, Saint Patrick doesn’t own his cathedral; he is associated with it, because it is dedicated to him. A dog doesn’t own its collar or the bowl it eats from, but the dog’s collar and the dog’s bowl are items that the dog obviously has a close association with!
St Patrick doesn’t own his cathedral and dogs don’t own their collars or their bowls
While there are many cases of association (or possession) that are clear, others fall into a grey area. And the unhelpful (or possibly helpful) news is that there is no single correct answer. Is it the girls’ college or the girls college? The boys’ club or the boys club? Schools and colleges are one category of things where the trend is to leave off the apostrophe; they are a case where girls is being regarded more as a noun acting as an adjective, rather than a noun showing possession. In other words, we are tending to regard girls school as being like a new school; a simple adjective, no apostrophe required. Clubs and other organisations fall into the same category, but note the difference between adults and children: we are happy to say boys club, but not men club. If you are talking about a specific school or club or other organisation, you just have to check whether that actual organisation uses an apostrophe or not: Presbyterian Ladies’ College, the Builders Laborers’ Federation, Melbourne Writers Festival.
Particular style guides and reference works may make their rules about which expressions do or do not use possessive apostrophes. For example, the travel-guide publisher Lonely Planet made a rule that, in its books, the terms artists colony, farmers markets, readers letters and travellers cheques would not take apostrophes. That doesn’t mean other people have to refrain from using apostrophes in those terms: that was just a decision Lonely Planet made about its own publications.
Farmers market and readers letters: you make your own decision about whether to use an apostrophe or not
Time is another category where the use of apostrophes is changing. It used to be that all expressions like a day’s work, two weeks’ holiday, three months’ extension all took apostrophes. Now the trend is that plural expressions of time don’t have apostrophes but singular expressions do: two weeks holiday, three months extension but a day’s work, a month’s leave, one year’s figures. There is no logic to this; it is just how the English language is evolving.
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