The lower the tail, the longer the back. A longer back is more important than depth of keel.
All other things being equal, choose the longer back rather than the longest keel.
I did not make this up, it comes from a 1921 9 page booklet by Sussex Judge William White Broomhead.
A Sussex breeder and Judge back when the Sussex was raised for its meat. He later when
on to become President of the British Poultry Club.
http://babel.hathitrust.org/cgi/pt?id=coo.31924003137332;view=1up;seq=5
This is an excellent reference regardless of the variety one is breeding.
It deals with the general type and
why it is so.
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The pinched tail is indicative of a narrow rear end. You want breadth of body
for egg production sake and to make room for the reproductive organs. You can have a whipped tail in a body of proper width. The phrase "knows she has a liver." means the birds eats more than it needs to for the egg it produces.
Best,
Karen