Rules of Eternus (Planar)

Prone Fighting: When you're prone, you're lying on the ground on either your front or back. You cannot take a stance except for crawling. As a move you can crawl 5 feet while prone as a move action, or you can stand up.

Instant Stand (Free): You can stand up from a prone position as a free action once per round.

Common or No Terrain: T-75
Uncommon Terrain: T-100