after reflection on the subject and looking thru a glass of Bushmills , I'm thinking that pub measures (optics) of spirits were based on a fraction of a gill (1/4 pint) , the English measure 1/5th of a gill (20 to the pint) and the Scots measure 1/4 of a gill (16 to a pint) So how a 1/2 of a pint of Beer became known as a gill I have no idea .
