This subsection deals with singular nouns pronounced with a sibilant sound at the end: /s/ or /z/. The spelling of these ends with -s, -se, -z, -ze, -ce, -x, or -xe.
Many respected sources have required that practically all singular nouns, including those ending with a sibilant sound, have possessive forms with an extra s after the apostrophe. Examples include the Modern Language Association and The Economist. Such sources would demand possessive singulars like these: Senator Jones's umbrella; Mephistopheles's cat. On the other hand, some modern writers omit the extra s in all cases, and Chicago Manual of Style allows this as an “alternative practice”. Generally, Chicago Manual of Style is in line with the majority of current guides, and recommends the traditional practice but provides for several exceptions to accommodate spoken usage, including the omission of the extra s after a polysyllabic word ending in a sibilant. Rules that modify or extend the standard principle have included the following:
* If the singular possessive is difficult or awkward to pronounce with an added sibilant, do not add an extra s; these exceptions are supported by The Guardian, Emory University's writing center, and The American Heritage Book of English Usage. Such sources permit possessive singulars like these: Socrates' later suggestion; James's house, or James' house, depending on which pronunciation is intended.
* Classical, biblical, and similar names ending in a sibilant, especially if they are polysyllabic, do not take an added s in the possessive; among sources giving exceptions of this kind are The Times and The Elements of Style, which make general stipulations, and Vanderbilt University, which mentions only Moses and Jesus. As a particular case, Jesus' is very commonly written instead of Jesus's – even by people who would otherwise add 's in, for example, James's or Chris's. Jesus' is referred to as “an accepted liturgical archaism” in Hart's Rules.
Similar examples of notable names ending in an s that are often given a possessive apostrophe with no additional s include Dickens and Williams. There is often a policy of leaving off the additional s on any such name, but this can prove problematic when specific names are contradictory (for example, St James' Park in Newcastle [the football ground] and the area of St. James's Park in London). For more details on practice with geographic names, see the relevant section below.
Some writers like to reflect standard spoken practice in cases like these with sake: for convenience' sake, for goodness' sake, for appearance' sake, for compromise' sake, etc. This punctuation is preferred in major style guides. Others prefer to add 's: for convenience's sake. Still others prefer to omit the apostrophe when there is an s sound before sake: for morality's sake, but for convenience sake.
Personally, I think "-s's" looks better when we're not dealing with plurals, and I think it's more important to clarify whether we are dealing with a swarm of people named Roxa, than to clarify exactly how that last "s" is pronounced. I would say we should adopt "-s's" as our style, and make sure to clarify other "Not wrong but not how we do it" things in the MoS.