The use of metal coins as a means of exchange and purchase goes back more than 2,500 years to ancient Greece, and was one of the key factors in helping Athens develop an empire in the years before Roman dominance of the Mediterranean. It was around this time that standardized coinage was introduced, each coin being worth a fixed amount, and valued the same as a coin of an identical nature, based on its size, weight, and the official marks or stamps which indicated its value. Metal had always been preferred as a means of coinage, since it was portable, long-lasting, and easily divisible, and also had the advantage of carrying its value within the coins themselves. Gold and silver were the favored metals because they were rare, easy to identify, and visually attractive, but they had a disadvantage in that they could be manipulated. A common practice was the removal of a small piece of the coin, known as "clipping," to get the precious metal, which could then be recycled: experts could do this without there being any noticeable change in the size or shape of the coin. This, among other things, eventually led to the development of representative money, where coins were changed from a unit of weight of precious metal to a unit of value, where cheap metals were used, and where, since the coins had little intrinsic value, clipping would not benefit the holder. The next step in the evolution of money was the use of paper money in the form of the bank note, or bill. This came about when banks started issuing a paper receipt to their customers, who had deposited coins and precious metals with them for safekeeping. These receipts stated that they could be exchanged for a given weight of precious metal (the British pound, for example, could be exchanged for a pound of silver). So many of these receipts were eventually in the hands of the public that people began trading them as actual money.