Obviously the most common punctuation mark, periods are used to end sentences. But also be conscious of how periods can and should (and sometimes should not) be used in abbreviations and acronyms. Also recognize that, while not technically grammatically correct, periods can create fragments for rhetorical effect (consider, for example, a slogan that says, “The. Best. Food. Ever.”–what do those periods do? They add emphasis.)
Period difficulty ranking: 11/14 (pretty easy to master!)
Periods seem like they should be the easiest punctuation mark to master since we use them the most often, but there are a few rules and tricks with them, such as using them for fragments with only certain acronyms. Learn the period usages so that you don’t make a mistake on what seems like the simplest!