Is it "The case' closure" or "The case's closure"? How exactly do I pronounce it?
The possessive suffix has three pronunciations. Which one you use depends mainly on what the last sound is:
- /ɪz/, following sibilants (the six sounds /s/, /z/, /ʃ/, /tʃ/, /dʒ/, and /ʒ/)
- /z/, following voiced sounds other than sibilants (including vowels)
- /s/, following voiceless sounds other than sibilants
You might have noticed that these rules are exactly the same as the rules for pronouncing the regular plural suffix -(e)s. But the possessive suffix is a little more complicated, because sometimes it isn't pronounced at all.
When it is pronounced, it's typically written
When it's not pronounced, it's typically written
So how do we know when to pronounce it?
If the word ends in a sibilant and falls into one of the following categories, the possessive suffix is not pronounced, and is therefore written with just an apostrophe:
In these examples, the /z/ and /s/ are already there as part of the plural suffix -e(s), so we don't have to add another (and indeed shouldn't).
Irregular plurals where the plural affix itself is regular (for example leaf, where the base changes to leave, but the plural affix is regularly pronounced /z/):
In certain fixed expressions:
for goodness' sake
for conscience' sake
In Greek names of more than one syllable:
One more rule: If a name ends in a /z/ sound, it may or may not be pronounced:
Dickens's OR Dickens'
Jones's OR Jones'
Burns's OR Burns'
But although both versions work, people tend to say the former and write the latter more often.
Your example is the word case, pronounced /keɪs/. It ends with an /s/ sound, which is a sibilant, one of the six sounds listed at the top of this answer. That means we write the possessive
's and we pronounce it /ɪz/, making the possessive form case's /keɪsɪz/.
For more information, see A Comprehensive Grammar of the English Language, p.320. Most of my examples and rules are drawn from this book. Additional discussion can be found in The Cambridge Grammar of the English Language, p.1595. (Please note that the possessive form is often called the genitive form in linguistics, and technical references may use that term instead.)