Byr

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Byr was often used in geology and astronomy as a unit of one billion years. The "b" is an abbreviation for "billion" (i.e., thousand million), with "yr" simply an abbreviation for "year". Today, the International System of Units specifies to use Ga, though Gy or Gyr may be used sometimes. Since a billion means 1000 million in the U.S. but has meant a million million in Great Britain, and still does in many non-English speaking countries, its use is discouraged.