Option B9A : ” Easy Handling” Six Wheel Safelock System for larger ladders. This safelock system adds another pair of swivel casters to allow easy movement around corners and all directions.
U.S. Patent #6,305,496 B1
TILTNROLL® ladders feature a caster system with a special balanced design that allows the ladder to tilt easily to the rolling position.
U.S. Patent #5,082,086