All sizes are measured on flat surface. You can see them in the charts.
In case of brand goods ( Punk Rave, Phaze etc.), all sizes are measured by manufacturers. As a result they can differ (1-2 cm) from sizes in description.
When you want to buy a corset, the size depends on bones (flexible or steel )
If you choose flexible bones, it is better to order the corset that is 3-7 cm smaller in the waits.
If you choose steel bones, it is better to order the corset that is 5-12 cm smaller in the waits.
For more info move to CORSETS section.