The apostrophe is used before the letter s to mark a possessive (Sharon's new car, the horse's feet, the children's toys). It can cause problems when the last letter of the word to which it is attached is itself s. If the word is plural, the apostrophe is used on its own without another s (the girls’ shoes, the horses’ feet). If the word is singular, the usual modern practice is to add 's as normal if there is an extra s in speech (Bridget Jones's Diary, Jesus's words to the Apostles). It is not incorrect to add a simple apostrophe in such cases, and this is usual if there is not an extra s in speech (Manasses’ prayer, Socrates’ submission to the law). Note that an apostrophe should not be used before s to make ordinary words plural (the so-called ‘grocer's apostrophe’, as in apple's 40p a pound). It should only be used to make plural forms of letter and numbers (a's, P's, 4’s) and is not always needed then (1940s)