Back Leg

The leg that is back and furthest from your direction of focus. For example, if you are standing in Heiko Dachi facing an opponent at angle 1, then stepping back with the right leg will move it further away from your opponent and make your right leg the Back Leg.

There is neither a Front Leg nor Back Leg if you are in a parallel stance such as Heiko Dachi (unless an opponent is positioned somewhere other than directly in front or behind you).