Here is a line ad from TCU...

Petite Polynesian Princess. 5'5", 100#, size 0 waist. Msg/Dom Fant/Live shows

Now my question, can you have size 0 waist, and if so, how do you measure it?

RW

Very carefully, and since there is nothing there, you may need some army night vission goggles to spot a piece of lint or dust?Just goes to show, read the TCU line adds with some degree of skepticism?

Yes, in women's there is a size zero.  The rule to thumb for measurement is to add 20 to the jean size, so her waist would be 20 inches.  If a girl wore size 9, her waist would be 29 inches, etc.  (I used to work at a jean store, that's how I know, and now you know!)