Although the practice is acceptable today, polite society in 18th-century America did not tolerate those who played with their genitives. Especially in public. The faux pas was so serious that a violator could be forever banished from intellectual circles.
This was so because a gaggle of 18th-century grammarians—dilettantes who had butlers to brasso their buckles and maids to press their jodhpurs—had arbitrarily decreed: “From henceforth and forevermore the genitive form shall mean only the possessive form and the genitive marker shall be ‘s, and whosoever shall use this possessive form with inanimate objects shall be made to bear a scarlet 'S.”
We use the genitive to express a relationship between nouns. It comes in two forms: the of (or genitive) form—“the purpose of the man,” “the mast of the ship”—and the ’s (or genitive/possessive) form—“the man’s purpose,” “the ship’s mast.”
The dilettantes reasoned that even a fool knows that ships are inanimate and therefore cannot possess anything, so the unenlightened caught writing “the ship’s mast” could find themselves not invited to the next garden party.
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.