You know how picky I can be, right? Eler Beth was doing some geometry today, and I reminded her that you always put a period at the end of the abbreviation for "inch". And then I reminded her that you never use a period at the end of any metric abbreviation. She was writing "cm", so I said (with emphasis), "Never, ever, put a period after 'cm'."
She answered, "What! Will they sue me if I do?!"