10/6    When to use apostrophe? Is there anything you haven't
        learned already?          http://apostrophe.me
        \_ I already know all that. I could use some coaching on the use
           of commas though.
        \_ This is so short.  I wonder about other usages like:
           "I have three PCs, and they run three different OS's.   One PC's
           OS's name is Linux, and the other two PCs' OS's names are not
           Linux".
           Is the above usage correct?
           \_ Generally, OSes and PCs are the preferred forms.  I'll only use
              an apostrophe if the construction is ambiguous otherwise.  (Such
              as the single-letter exception noted on that page.)  -tom