In the natural gas industry, where quantities are often expressed in standard cubic feet, large multiples of standard cubic feet are generally not expressed with metric prefixes, but rather with prefixes based on roman numerals, where the "s" for "standard" is often omitted. Common units of gas volumes include ccf (hundred standard cubic feet), Mcf (thousand standard cubic feet), and MMcf (million standard cubic feet). The "M" refers to the Roman numeral for thousand, while two "M's" represent one thousand thousands, or one million. Bcf (billion standard cubic feet), Tcf (trillion standard cubic feet), Qcf (quadrillion standard cubic feet), etc., are also used.