Singular nouns that end in s present special problems. Imagine that your last name is Woods (and you teach English grammar). Your name is singular, because you are only one person. When students talk about you, they may say,
Ms. Woods’s grammar lessons can’t be beat.
Ms. Woods’ grammar lessons can’t be beat.
Both of the sentences about you and your grammar lessons are correct. Why are there two options? The answer has to do with sound. If you say the first sentence above, by the time you get to the word grammar you’re hissing and spitting all over your listener. Not a good idea. The second sentence sounds better. So the grammar police have given in on this one. If the name of a singular owner ends in the letter s, you may add only an apostrophe, not an apostrophe and another s. But if you like hissing and spitting, feel free to add an apostrophe and an s. Both versions are acceptable.
Which sentence is correct?
A. The walrus’ tusk gleamed because the walrus brushed it for ten minutes after every meal.
B. The walrus’s tusk gleamed because the walrus brushed it for ten minutes after every meal.
Answer: Both are correct. Sentence B calls for more saliva, but it follows the rule. Sentence A breaks the old rule, but nowadays breaking that rule is acceptable. (Yes, it was a trick question. You know how teachers are.)
Try another set. Which sentence is correct?
A. My whole family got together for Thanksgiving. The Woods’ are a large group.
B. My whole family got together for Thanksgiving. The Woodses are a large group.
Answer: Another trick question. Sentence B is correct because Woodses is a plural, not a possessive. In sentence A, the apostrophe is incorrect because plurals shouldn’t have apostrophes unless they express ownership.