A bit of random trivia about the English language follows. It will probably bore the average reader to death, but I find it very interesting. The germ of the following discussion comes from A History of the English Language by Cable and Baugh.
In English, a possessive noun (called a genitive by grammar snobs) is indicated with an apostrophe and the letter ‘s’. Thus, “Tim’s book”, for example. Why is it done this way? The only other use of the apostrophe in the English language is to signal elided sounds (or letters) in speech (or writing), such as in contractions like “can’t” for “can not.” The possessive in English uses the apostrophe for the exact same reason, as it turns out. But what could possibly be the elision?
It is the word “his,” oddly enough.
In Middle English (spoken around the time of Chaucer, in the last half of the 14th century), the possessive was formed by adding an ‘-s’ or an ‘-es’ suffix to a word. Thus rather than “Tim’s book” it would have been “Times Boke” (book was spelled, and pronounced, differently then, too). “Stone’s” would have been written “stones.” Note that the “-es” ending in “Times” or “stones” is pronounced in Middle English: the “e” is not silent, and it is the vowel of the second syllable in both words. Middle English “Times” is not prounced like modern English “times,” rather it is the syllable “Tim” with an “es” syllable after that: so “Tim-is”. “Stones” is similarly two syllables (“stone-is”), and in general, the “-es” suffix was another syllable at the end of the noun that it made possessive, rather than just an “s” sound on the end of an already existing syllable.
Old English was a heavily inflected language, but stress was on the first syllable of a word. Inflections are on the end of a word (an inflection is an added morpheme, like the suffix “-es,” which signals how the word is being used, such as whether it is possessive, or plural, or past or future tense, etc.). Unstressed syllables were not very carefully pronounced, especially by the time of Middle English.
It was thus not uncommon for the “-es” genitive ending to be pronounced like “is” or “ys.” As fate would have it, the word “his” in Middle English often had the initial sound of the “h” left off, so that the genitive “is” and “his” were pronounced exactly the same. And that fact is at the root of why we use the apostrophe with the “s” for a possessive in English. Since “Times” (that is, “Tim-is”) and “Tim his” were pronounced the same, people thought they were spelled the same (spelling standards were much less strict in Middle English). So, some authors even wrote out “Tim his” in their texts, rather than “Times.” Indeed, Cable and Baugh list a couple instances of Shakespeare using this construct: “’Gainst the count his galleys I did some service” (where “Count his” would be “Count’s” today), and “In characters as red as Mars his heart,” where “Mars his” would be written “Mars’” or “Mars’s” today)1.
Now, this is a written oddity. No one actually pronounced “Mars his” or “Count his,” as we would today, it was simply “Mars is” then. But, since the “-es” suffix is unstressed on the end, it’s quite natural to contract the “es,” or “is” sound in to simply an “s” sound. Thus, once it became popular to write “Tim his books” but pronounce it as “Tims books,” the natural thing was to use the apostrophe to indicate the elision and save some writing, thus “Tim’s” books.
That’s how we use the apostrophe today: it’s descended from a “mistake” (as much as anything in a completely arbitrary language can be a mistake). This fact always makes me laugh a little bit at the language police that go crazy over the misuse of the possessive apostrophe, as the use of the apostrophe itself is merely a commonly accepted “misuse.”
1Cable and Baugh, page 241.