J.M. NORTHUP

The general rule for singular nouns is to use an apostrophe and s at the end to show possession. EXAMPLES: the teacher’s, Mom’s, and Sahara’s.

The general rule for regular plural nouns is to use an apostrophe only at the end to show possession. EXAMPLES: the teachers’ lounge, the racers’ cars, and the singers’ voices.

Purdue Online Writing Lab had this note, but it is not what I found to be true during my research:

The generally accepted convention for most academic styles (including CMOS, APA, and MLA) is to add apostrophe + s to the singular form of the word, even if it ends in “s.” Non-academic styles, such as AP, suggest simply adding an apostrophe to the end of a word that ends in “s.” Please check the style guide of whatever format you’re using to make sure you’re in line with their recommendations.

https://owl.purdue.edu/owl/general_writing/punctuation/apostrophe_introduction.html

What I found from most universities, style guides, and manual resources was what I was taught (and do), which clashes with the Chicago Manual of Style (CMOS 7.15–28) when it comes to nouns ending in s (whether singular or plural). We says possession is shown by using an apostrophe only. EXAMPLES: Jewels’ coat, Thomas’, and Alexis’.

BEST ANSWER:

It’s the age-old question: Should you add apostrophe + “s” to singular words that already end in “s” (such as James, cactus, or dress), or just an apostrophe? The answer is that both solutions are correct. It depends on your style guide and your personal preference. 

• The class’ pet bunny was missing. (Correct)
• The class’s pet bunny was missing. (Correct)

https://www.yourdictionary.com/articles/apostrophe-rules

These examples show you that it really all depends, which is why it gets confusing:
Most of these say not to use ‘s after s unless it is a proper name? That is a new spin on what was just said, huh?

The Punctuation Guide was a great resource for explaining things. It gave the general rules and the exceptions. Of course, the easiest one I read was from The Britannica Dictionary.

Do not create a double or triple “s” when adding an apostrophe

If adding an apostrophe “s” would result in a double or triple “s” at the end of the word, erase the “s” after the apostrophe and leave the apostrophe in place. Example:

• Sophocles‘ house, NOT Sophocles‘s house
• Several bosses‘ houses, NOT several bosses‘s houses

Note: Some grammar textbooks recommend keeping the “s‘s” or “ss‘s” ending for singular words or for names ending in “s” or an “s” sound because this spelling allows for easier pronunciation.Examples:

• James‘s party, Marx‘s teachings, the boss‘s house, the class‘s work, and congress‘s motion

https://guides.lib.uoguelph.ca/c.php?g=696372&p=4999533

WHAT IT BOILS DOWN TO:

Now, I cringe every time I see s’s, but it doesn’t make it wrong. The truth is, like with most writing, you just need to stay consistent in your choices.

There are some other rules to think about. Here are some great resources to help you understand them: