Actually Kevin, 3.2 to 5.6 would be ABV which is the percentage of Alcohol by Volume. This is now used instead of Original Gravity as a result of EEC legislation. O.G. would be a figure like .1042 which is about 4%ABV and would represent the specific gravity of the brewing wort, before fermentation.

Proof spirit is yet another way of measuring alcohol content, 70% proof is about 40% ABV. Originally, proof spirit was determined by pouring a specified measure of the spirit over a specified volume of gunpowder. The mixture was then ignited and if the gunpowder eventally detonated, the spirit was deemed proof. It was historically used as a way of deciding the degree of excise duty (ie proof or under proof) to be paid and was not a quantative measure of the spirit's strength.