Dovetail Jaws have been designed for mounting bowls by expanding into a recess or clamping onto a spigot.

The sizes indicated on the drawing show the Jaw measurements at a perfect circle.  VM120 and VM150 chucks provide approx. 48mm of expansion beyond these specified sizes shown with the exception of the 85mm Dovetails. All Dovetail Jaws have an internal dovetail angle of 13 degrees for clamping and an external dovetail angle of 13 degrees for expanding. 
 
The Vicmarc range of Dovetail Jaws also have a machined taper on the face of the dovetail which aids in clamping both on a tenon (clamp mode) and in a recess (expansion mode).  When clamping on a tenon it is often the case that the flat section of the bowl/tenon will be tapered the wrong way slightly or may have wood fibres or debris that can be in the corner of the tenon.  With the jaws being slightly tapered it allows for a firm and accurate hold slightly away from the corner.  This will ensure an accurate hold without wobble.  When clamping in a recess the flat section is often tapered towards the centre of the bowl.  This taper allows for a firm hold right in the corner (outermost diameter) which will ensure an accurate hold.
 
Above last image shows arrow pointing to the slight taper (Same on all Dovetail Jaws)