An apostrophe is the same character as a single closing quote: '. Confusion can sometimes arise when InDesign second guesses you and thinks you want an opening single quote rather than an apostrophe. This is most likely to occur in phrases like rock 'n' roll and fish 'n' chips, or in dates where the apostrophe substitutes for missing numerals, like '90s instead of 1990s.
Figure 6.5. Apostrophes.