 However others prefer to avoid this method (which can lead to confusion with the possessive "-'s"), and write "1990s", "buts"; this is the style recommended by "The Chicago Manual of Style".Likewise, acronyms and initialisms are normally pluralized simply by adding (lowercase) "-s", as in "MPs", although the apostrophe is sometimes seen.Use of the apostrophe is more common in those cases where the letters are followed by periods ("B.A.