There are a few special instances in which it is acceptable to use an apostrophe to indicate plurals, as with letters and symbols where s added without punctuation could look odd or be undecipherable (dot your i’s and cross your t’s; he rated a string of 9's.9 from the jury). However, when forming plurals of regular nouns it is wrong to use an apostrophe, e.g. six pens not six pen’s. For further discussion, see apostrophe¹ (usage).