Consulting a dozen or so recently published punctuation guides, I can report that they contain minor disagreements on virtually all aspects of the above and that their only genuine consistency is in using Keats's poems as the prime example. Strange but true. They just can't leave it alone. "It is Keats' poems (NOT Keats's)," they thunder. Or alternatively: "It is Keats's poems (NOT Keats')." Well, poor old Keats, you can't help thinking. No wonder he developed that cough.
Having said that there are no absolute rights and wrongs in this matter, however, when many people wrote to ask why St Thomas' Hospital in London has no "s" after the apostrophe, I did feel that the answer much echo Dr Johnson's when asked to explain his erroneous definition of a pastern: "Ignorance, madam, pure ignorance." Of course it should be St Thomas's Hospital. Of course it should.
There are three basic rules, which I won't mention here. When a noun such as Keats is possessive, the lone apostrophe is fine and follows the rule. When the extra syllable is added, the extra s (Keats's is okay (Keats-uz).
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Cruelty to punctuation is quite unlegislated: you can get away with pulling the legs off semicolons; shrivelling question marks on the garden path under a powerful magnifying glass; you name it.