text: why the double 'b' in barrel per day? when quoting barrels of oil.  the common notation is 'bbl'.  does anyone know the reason for the double 'b'?  thanks. blue barrel  in reference to petroleum, specifically oil and natural gas liquids, bbl is an abbreviation for 1 barrel, equal to exactly 42 u.s. gallons, or approximately 159 liters. the abbreviations 1 mbbl and 1 mmbbl are generally accepted to mean one thousand and one million barrels respectively. (in non-industry documentation mbbl can sometimes stand for one million barrels.) while north american markets typically measure oil in barrels, elsewhere oil is more commonly measured in cubic meters and less commonly in metric tonnes. natural gas is measured differently.  in the early 1860's, when oil production began, there was no standard container for oil, so oil and petroleum products were stored and transported in barrels of all different shapes and sizes (beer barrels, fish barrels, molasses barrels, turpentine barrels, etc.). by the early 1870's, the 42-gallon barrel had been adopted as the standard for oil trade. this was 2 gallons per barrel more than the 40-gallon standard used by many other industries at the time. the extra 2 gallons was to allow for evaporation and leaking during transport (most barrels were made of wood). standard oil began manufacturing 42 gallon barrels that were blue to be used for transporting petroleum. the use of a blue barrel, abbreviated "bbl", guaranteed a buyer that this was a 42-gallon barrel.