HP TextEngineer Technical Writing Guide - Manual (Page 18)

Company Writer's Guide
[For Illustration Only] B-3
3. Abbreviations with periods, lower-case letters used as nouns, and capital letters that would
form another word if “s” alone were added form the plural with an apostrophe and an “s”:
Ph.D.’s
x’s and y’s
S’s, A’s, I’s
Letter Symbols for Units of Measurement
The following pages are taken from Corporate Standards. These abbreviations should be the only
ones used unless stated otherwise in a project-specific document.