I believe then when a proper noun that ends in the letter "s" (such as Bayes or Charles) is written with an apostrophe at its end, you pronounce the proper name and then a sound that's very similar to how you pronounce the word "is." You pronounce the "is" sound right after you say the proper noun as if it were an extra syllable.
Thus to pronounce Charles' you say, "Charles-is." To pro
The basic rule given in most modern style manuals for singular names like Bayes and Charles is to add 's. You will also find in some style manuals, usually the older ones, the rule that singular names ending in s should add only the apostrophe. In older texts, therefore, you will see things like Bayes' and Charles'. These should be pronounced the same as Bayes