The self-leveling pendulum ensures a level line. You know your line is out of level when it blinks. Once level, the line goes solid. Lock the pendulum to set out angled lines or to protect the mechanism when you aren’t using it.The Leica Lino L4P1 runs on both rechargable Li-Ion batteries — good for up to 24 hours on a single charge — as well as standard alkaline batteries.