Step Jaws, designed by the famous Australian wood turner and author Richard Raffan, are used for holding a range of spigot sizes with minimum jaw movement and without the necessity to change the jaws. These Jaws are designed primarily for bowl turning and they are especially useful when roughing out a number of bowls at one time. Boxes are also easily turned using the step jaws. The bottom can be turned by holding the rim without leaving unsightly pressure marks.
The sizes indicated on the drawing are when the jaws form a perfect circle.  VM90 and VM100 chucks provide approx. 38mm of expansion beyond these specified sizes.