Oil companies that are listed on American stock exchanges typically report their production in terms of volume and use the units bbl (one barrel), or kbbl or Mbbl (one kilobarrel, one thousand barrels), or MMbbl (one million barrels), and occasionally for wider comprehensive statistics Gbbl (or sometimes Gbl), for giga-barrel (one billion barrels). There is a conflict concerning the units for oil barrels (see ). For all other physical quantities, according to the International System of Units, the uppercase letter "M" means "mega-" ("one million"), for example: Mm (one million metres, megametres), MHz (one million hertz, or megahertz), MW (one million watts, or megawatt), MeV (one million electronvolt, or megaelectronvolt). But due to tradition, the Mbbl acronym is used today meaning "one thousand bbl", as a heritage of the roman number "M" meaning "one thousand". On the other hand, there are efforts to avoid this ambiguity, and most of the barrel dealers today prefer to use bbl, instead of Mbbl, mbbl, MMbbl or mmbbl.