How do you use the possessive apostrophe after Alex? Alex's or Alex'?

I recall in school learning that if a proper name ends in an S or in the sound of an S like in Alex, put the apostrophe after the last letter. Is this correct?

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MBAdomaitis profile image83

Mary Beth Adomaitis (MBAdomaitis) says

4 years ago
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diogenes profile image83

diogenes says

6 years ago
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