. . . "A pound coin originally weighed one troy pound of sterling silver, giving the currency the name "pound sterling". "Sterling silver" means mixed metal that has 92.5% or more real silver. One pound sterling was originally divided by 240 sterling pence. This was because there are 240 pennyweights in a troy pound." (Wiki)Sterling silver is slightly less pure than Britannia silver (95.8% Ag), but is slightly harder than it, and thus articles made from sterling silver are more durable.
Don't confuse Britannia Silver with Britannia metal, which is an alloy of approximately 92% tin, 6% antimony, and 2% copper, very similar to pewter.
It is thought likely that sterling is derived from the old Norman-French "steorling", or "coin with a star".
