Generally, the distance from the bottom of the post to the slot that holds the saddle rail. When you're talking about something that's 30-40 cm long (that's 9-10 inches), it's point less to quibble about a couple millimeters. Shorter posts are generally for bikes with horizontal top tubes. People who ride bikes with slanted top tubes (or who ride bikes that are too small for them) choose longer seat posts. When measuring for your fit, be sure to use the minimum insertion point, etched a few centimeters up from the bottome.