Word of the day (unsurprisingly) - "metastasis"
What did I tell you about tumours, Gabriel. Gah.
Anyway, I rummaged through an old OED in my librarium, and there appear to be two somewhat contradictory principles when dealing with the possessive apostrophe.
a) The euphony principle; if the additional "-es" syllable turns the word ungainly then you should put the apostrophe outside - eg. hangers', cleaners', users', executives' but words ending in a sibilant like "chess" or "James" in which the added syllable doesn't sound so bad (viz. "chess-es", "James-es") should have the 's added. To answer your question, Gabriel - the possessive of Jess would be "Jess-es" in this case.
However, in the wonderful world of English, it seems to me that the euphony principle may lead to weird instances like synopsis's (si-nop-sis-sus) vs synopses' (si-nop-sus).
b) However, the OED also ruefully notes that words with a silent "s", usually of French origin, are often accorded the apostrophe outside (Dumas', corps') due to traditional historical usage, despite the euphony principle. Also already mentioned that classical, literary or Biblical names only use the apostrophe outside due to tradition - Isis' or Dickens' - even though Isis's isn't (THAT) euphonically unpleasing.
In other words it all freaking depends.
The OED concludes by arguing lamely that the best you can hope for is to be internally consistent throughout any single text or your general writing style:) It also suggests (as do many other style or copywriting manuals) that one rephrases the possessive using "of" (the hotels' owner into "the owner of the hotelss") or use the attributive form (the hotels owner).
Below are some fascinating professional online takes on the topic.
How is the possessive formed for a word like "McDonald's"?
Comments: The results include one divided vote: the voter agreed with position 1 for formal writing, but position 2 for informal writing.
POSITION 1 (10 votes): Rewrite.
EXPLANATION: An apparent need for the possessive form of the name of the fast-food chain is conceivable; for example, the executives of the company might want to refer to the company's (note the possessive) employees or products.
The most commonly accepted way to form the possessive singular is to add "'s"; some prefer to add only "'" to singulars that end in a sibilant. Forming the possessive of "McDonald's" in those ways produces "McDonald's's" and "McDonald's'"; neither looks like English, and both would have to be considered nonstandard or unpronounceable.
A sentence that seems to require such a possessive form should be recast to yield the "of" possessive or should use the noun as a modifier. For instance, "The golden arches of McDonald's (meaning the company) are common" or "McDonald's golden arches are common."
POSITION 2 (7 votes): This can only arise colloquially, or in artificially contrived situations. For those cases I favor McDonald's.
EXPLANATION: For a possessive form like McDonald's to need its own possessive, it must have become the nominative form of a different noun.
While I may say "I went to McDonald's for a Whopper," what I mean is I went to the McDonald's fast food store. McDonald's is a trademark, so is thus an adjective, not a noun.
When you reply "McDonald's burger is called a Big Mac," you mean the McDonald's fast food store's burger is called a Big Mac. When you instinctively say and I immediately understand "McDonald's burger," we are temporarily creating a McDonald in our minds to personify the McDonald's fast food store.
POSITION 3 (1 vote): McDonald's'.
EXPLANATION: I would make the possessive McDonald's'. But I'm considered weird. McDonald's' is logical. So what if there are two apostrophes? The basic word, the very name of the business, is McDonald's. If you're talking about something that belongs to McDonald's, such as McDonald's' environmental philosophy, it's McDonald's'. Isn't it?
(Where) should there be an apostrophe in "Veterans" in "Veterans Administration" and similar constructions?
Comments: Opinion was nearly equally divided, with position 1 (no apostrophe) winning by 2 votes.
POSITION 1 (7 votes): No apostrophe in that and many other, but not all cases.
EXPLANATION: The distinction is between a possessive, as in user's guide, and a plural noun acting as an adjective, as in writers union, tinkers guild, and veterans administration.
In the case of a user's guide, the item is clearly possessed by the user; it doesn't describe, explain, or otherwise touch on properties of the user.
The Veterans Administration deals with the concerns of veterans. It does not belong to any particular veteran, the generic veteran, or the class of veterans.
Similarly, the writers union doesn't belong to writers. In fact, the very opposite is true. :-)
I would also say "children's club," but I think this falls into the same category as "user's guide." The focus is on individual children and their venues, like their rooms or their back yards. We look on children and (in a sense) users as immature.
Where the focus is more on the organization than the individual, as in a writers guild, this changes. I would say "Boy Scout troop" or "Boy Scouts troop," not "Boy Scout's troop" or "Boy Scouts' troop."
POSITION 2 (5 votes): Use a possessive form (singular or plural depending on meaning).
EXPLANATION: Such constructions are best understood as genitives, rather than possessives in the narrow sense. Thus a writers' union is a union of writers, not a union possessed by writers--but still gets an apostrophe.
In English, plural nouns are seldom if ever used as adjectives. Thus, we speak of a "shoe (sing.) store", even though the store clearly has more than one shoe for sale.
We can see that constructions like "writers' union" are genitives if we consider cases where the genitive and the simple plural are actually pronounced differently. We would certainly say "children's club" rather than "children club".
The Veterans' Administration itself is a moot point, since its official name has changed. But as an administration pertaining to veterans, the genitive plural is appropriate, and the name would be properly punctuated as "Veterans' Administration".