Many abbreviations have a period at the end, for instance, “Feb.” and “etc.”
Sentences also can have a period at the end. So what do you do when an abbreviation occurs at the end of a sentence? You might wonder if two periods are needed, one for the abbreviation and one for the sentence, e.g., “milk, bread, eggs, etc..”
In fact, when the term “etc.” ends a sentence, the Chicago Manual of Style states that one period is used.
Incorrect: “milk, bread, eggs, etc..”
Correct: “milk, bread, eggs, etc.”
In this case, the period performs two functions: it ends the abbreviation and it ends the sentence.
However, what happens when “etc.” occurs just before a question mark? After all, a question mark does contain a “period” underneath the squiggly bit.
The answer is that the period within the question mark cannot do double duty as a period for the abbreviation. In the English writing system, the question mark is a single unit that cannot be separated into its component strokes. You hence need a separate period for the abbreviation.
Incorrect: “milk, bread, eggs, etc?”
Correct: “milk, bread, eggs, etc.?”
This is true for exclamation marks as well.