See CMS 6.113–6.115 for general apostrophe usage.

See CMS 7.13–7.19 for the general rule on forming possessives, as well as exceptions to the rule. See CMS 7.29 for the general rule on contractions.

For plurals of variables, add apostrophe “s” (r’s, σ’s); for plurals of acronyms, just add “s” (GNPs), including acronyms that end in s (CDSs).