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  Roman numerals use different symbols for each power of ten, and there is no zero symbol, in contrast with the place value notation of Arabic numerals (in which place-keeping zeros enable the same digit to represent different powers of ten).
  
  This allows some flexibility in notation, and there has never been an official or universally accepted standard for Roman numerals. Usage varied greatly in ancient Rome and became thoroughly chaotic in medieval times. The more recent restoration of a largely "classical" notation has gained popularity among some, while variant forms are used by some modern writers as seeking more "flexibility". Roman numerals may be considered legally binding expressions of a number, as in U.S. copyright law before the Berne Convention Implementation Act of 1988 (where an "incorrect" or ambiguous numeral in a copyright notice could invalidate a copyright claim or affect the termination date of the copyright period).