In Everyday English: Getting to Grips with the Basics of the Language, Michelle Finlay writes, “Despite its diminutive size, the apostrophe has managed to create havoc in written English.” Or, rather, we have created havoc with this punctuation mark, whose primary function is to denote possession, as in the “dog’s paws” (not to be confused with the “dogs’ paws” in the case of multiple dogs), or omission, as in the contractions “don’t” and “it’s.”
In response to this disorder, an Apostrophe Protection Society – believe it or not – was established in the United Kingdom in 2001, and its website is filled with real-life examples of misuse: “Its the law,” “Beauty care that let’s me be me,” “Public PC’s,” “Visitor’s please ring the bell for assistance,” and “East York Farmer’s Market: Tuesday’s from 8:00 am – 2:00 pm.”
One of the most common forms of confusion arises when describing one’s home, with mailboxes variously proclaiming The Smiths or The Smiths’ or The Smith’s. Assuming that more than one Smith is in residence, we can rule out the singular possessive (The Smith’s), though even a solitary Smith would not be preceded by “The” unless he had delusions of grandeur, headed a Scottish clan (e.g., The Macnab), or, as one editor put it, “employs hammer, tongs, and anvil to practice her livelihood.”
In the case of more than one Smith, much depends on one’s intent. When stressing ownership, The Smiths’ would be preferable, since this implies that the mailbox and, by extension, the home associated with it belong to the Smiths. But if one simply wants to say that a family called Smith resides at this address, then The Smiths will do, though a purist might assert that the Smiths are more than the sum of their possessions.
So use the apostrophe with care, excluding it from possessive pronouns, such as its or theirs, and from all plurals except in the service of clarity, as when dotting one’s i’s and crossing one’s t’s – always an excellent thing to do where the English language is concerned.