The Law of One Price is a fundamental implication of traditional microeconomic theory. It states that an item sold in a well-functioning market cannot have more than one price. However, articles likely in the same market can have different prices if they are different. For example, a 500 MHz Macintosh G4 computer may have a different price to a 450 MHz Macintosh G4, but both are probably sufficiently close substitutes to be considered part of the same market. Equally, similar items sold in different markets may have different prices. For example, a 500 MHz Macintosh G4 in Melbourne, Australia may have a different price to the same machine in London, England as one is unlikely a good substitute for the other.
In traditional finance theory, the law is expressed as a basic assumption—no arbitrage—rather than as an implication.