How to Form a Joint or Compound Possessive
Today, playing with our genitives is acceptable because even the intellectual elite among us do not know that the possessive began long ago with something called the genitive. Here’s the simple rule now:
You may use the genitive/possessive form even with inanimate objects, like ships, unless it sounds awkward.
James Fernald writes in English Grammar Simplified, “The use of the inanimate possessive seems to be determined more by euphony than by logical prescription.” Because it is difficult to say “such risk’s likelihood,” it is better to write “the likelihood of such risk.” But either “the rays of the sun” or “the sun’s rays” is correct.
If two or more people possess an item or items together, we add ‘s only after the last person. Let’s say that George and Giuseppe together buy a 1949 MGTC; we then describe the antique car as:
George and Giuseppe’s ’49 TC
If George already owns a British racing green ’49 TC, and Giuseppe buys a turquoise ’49 TC, so their ownership of the cars is separate, we describe the cars as:
George’s and Giuseppe’s ’49 TCs
But if George and Giuseppe together own a British racing green ’49 TC, and the two together buy a turquoise ’49 TC, we again add ‘s only to the latter name:
George and Giuseppe’s ’49 TCs
Compound nouns contain two or more words, often separated by hyphens, but always read as one unit. Form their possessive and the possessive of any “closely associated” words by adding ‘s to the end:
my brother-in-law’s boat
the National Institute of Science’s recommendation
Sometimes we combine the genitive of construction with the genitive/possessive ‘s construction: a tic of Lucretia’s; a habit of Susan’s. Why this is acceptable used to confound me; shouldn’t we write a tic of Lucretia, and a habit of Susan? But think of it this way: If we wrote with the of construction using pronouns, which form of the pronoun would we use? The possessive: a friend of mine, not a friend of me; a book of his, not a book of him. So when a noun appears after the of, it too must be in the possessive form: a client of the firm’s.