Back in the swirling mists of Anglo-Saxon England, before the apostrophe punctuation mark was even imagined, nouns took on the possessive (or genitive) case without any punctuation at all. The same can be seen in German today.
A large number of nouns would end with the –es to show the possessive declension (e.g. þæs cyninges scip meaning the king’s ship), though not all nouns, as there were different endings for different genders.
This ending was carried over into Middle English and used far more widely among almost all nouns, but by the time Modern English arrived, the e was no longer pronounced as carefully and an apostrophe began to appear in its place.
So, we can blame those 16th-century scholars for all these grammar headaches.
For some reason, it was decided sometime back in the meandering history of the English language that the possessive version of personal pronouns could still manage without apostrophes.
Hers, its, ours, theirs and yours are all thriving without the extra attention.
But any indefinite or impersonal pronouns, it was decreed, should take apostrophes. So we have anybody’s, one’s, somebody else’s, etc.
Compound proper names and apostrophes
Generally, the apostrophe will go at the end of the phrase, e.g. the Prince of Wales’s tax bill or half an hour’s chat.
The situation becomes a little more complex if brackets are introduced (perhaps to give the name of the actor playing a character), as it is difficult to find an elegant position for the apostrophe.
For example, Darth Vader’s (Dave Prowse) lightsabre is one possible way to punctuate it, but so is Darth Vader (Dave Prowse)’s lightsabre.
There is in fact no agreed method to punctuate such phrases, so you can even punctuate both parts of the phrase: for example, Darth Vader’s (Dave Prowse’s) lightsabre.
It very much comes down to personal preference, which makes a nice change.
Another trap that awaits the unwary author is the false possessive.
These can generally be found where a noun ending in s has actually taken on a descriptive, adjectival role, but the s confuses the writer into thinking an apostrophe should be applied.
For example, That Super Furry Animals album is one of my favourites is quite correct, whereas The Rolling Stones’ tour schedule is punishing is wrong.
Re ‘Compound proper names’ and ‘False possessives’:
Please help me to differentiate between “the tax bill belonging to the Prince of Wales” and “the tour schedule belonging to the Rolling Stones”. Why do different rules apply?
Regarding the Star Wars example:
Darth Vader’s (Dave Prowse) Light Saber is more appropriate because Dave Prowse does not have a light saber, he has a prop that represents Darth Vader’s Light Saber. Is that only my opinion and not more of a logical reason?