فِي ٱللُّغَةِ ٱلْإِنْجْلِيزِيَّةِ:

x's

There has been some difference of opinion regarding the use of apostrophes in the pluralization of references to letters as symbols. New Fowler's Modern English Usage after noting that the usage fashion has changed states at page 602, "But after letters an apostrophe is obligatory." The 15th edition of The Chicago Manual of Style states at paragraph 7.16, "To avoid confusion, lowercase letters ... form the plural with an apostrophe and an s". The Oxford Style Manual at page 116 advocates the use of common sense.