anatomy of a jacobs chuck

The assembly of a Drill or Lathe where, during rotation, the Bit or material is held. A Chuck has movable jaws, for gripping Bit.

A Collet is a split-sleeve type of chuck -- but no movable jaws-- for holding drills and other such tools. Similar to a Drill's Chuck or Shaper's Spindle, an assembly on a Router where the Bit is inserted and gripped tightly. Uses a compressio sleeve, or split-sleeve,  to grip the Bit's Shank. The greater number of splits, also known as slits, the more efficiently the collet grips the Shank of a router's bit.

See [Anonymous], "Router Collets", Woodworking Magazine, Autumn, 2006, back cover, for info on a how a collet works, types of collets, and maintenance.

On the left, the anatomy of a Jacobs drill-chuck. (A) Body; (B) Jaws; (C) Split threaded ring, force fit into (D) Adjusting sleeve (Courtesy Jacobs Mfg. Co. Ltd.)

albrecht keyless chuck

On the left, the anatomy of an Albrecht keyless chuck. "The design enables even large drills to be gripped firmly when hand-tightened".

Source: Tubal Cain. Drill, Taps and Dies. Birmingham, England: Argus Books, 1976, page 38. Image courtesy Jacobs Mfg. Co. Ltd.)

Ordinarily, a Collet chuck is made to hold only one size of tool shank without the addition of an adapter.

Source: Home Craftsman 4 March-April 1935, page 172.

It is no use having a perfectly formed drill point if the drill itself is not held both firmly and true.

Source: Tubal Cain. Drill, Taps and Dies.Birmingham, England: Argus Books, 1976, page 38.

Sources :Home Craftsman 4 March-April 1935; Tubal Cain. Drill, Taps and Dies Birmingham, England: Argus Books, 1976.

By way of annotation on this book, I must say that it is one of the best books that I have ever purchased on an impulse at a used book store. It is 30 years old, is written in England -- which does not mean it''s "bad", simply that frequently books written in another country do not necessarily speak to your needs, because the products discussed are different, etc.