Conventional ladder having a pair of equal-length legs can stably stand on a flat floor, which however may be unstable to stand on a rugged surface, to even cause an accident due to collapse or slippery of the ladder. 
The ladder feet may be coated or packed with a rubber cushion to prevent slippery.  However, it still can not be used on a rugged surface.  If setting up such a ladder inclinedly to support on a vertical wall, the rubber cushion on each foot may only tangentially touch the floor surface, without fully scratching it, to thereby possibly cause a slippery accident. 
The present inventor has found the defects of a conventional ladder and invented the present ladder leveller.