For nearly a century, engineers used the abbreviation "MCM" to designate a "thousand circular mils" in specifying the cross-sectional area of large electrical cables. Since the mid-1990s, "kcmil" has been adopted as the official designation of a thousand circular mils, but the designation "MCM" still remains in wide use. A similar system is used in natural gas sales in the United States: "m" (or "M") for thousands and "mm" (or "MM") for millions of British thermal units or therms, and in the oil industry, where "MMbbl" is the symbol for "millions of barrels". This usage of the capital letter "M" for "thousand" is from Roman numerals, in which "M" means 1000.