or by picture. Most were round, but they are also found in square, heart or octagonal shapes. Thousands of towns and merchants issued these tokens from 1648 until 1672, when official production of farthings resumed, and private production was suppressed. There were again coin shortages in the late 18th century, when the British Royal Mint almost ceased production. Merchants once again produced tokens, but they were now machine made and typically larger than their 17th century predecessors, with values of a halfpenny or more. While many were used in trade, they were also produced for advertising and political purposes, and some series were produced for the primary purpose of sale to collectors. These tokens are usually known as Conder tokens, after the writer of the first reference book on them. These were issued by merchants in payment for goods with the agreement that they would be redeemed in goods to an equivalent value at the merchants' own outlets. The transaction is therefore one of barter, with the tokens playing a role of convenience, allowing the seller to receive his goods at a rate and time convenient to himself, and the merchant to tie the holder of the token coin to his shop. Trade tokens often gradually changed into barter tokens, as evidenced by the continued circulation of former trade tokens when the need for their use had passed. In the United States of America, Hard times tokens issued from 1832 to 1844 and Civil War tokens issued in the 1860s made up for shortages of official money. Because of weight, the U.S. Treasury Department does not ship coins to the Armed Forces serving overseas, so Army and Air Force Exchange Service officials chose to make pogs in denominations of 5, 10, and 25 cents. The pogs are about 38 mm (1.5816″ to be exact) in diameter, and feature various military-themed graphics. The collecting of trade tokens is part of the field of exonumia, and includes other types of tokens, including transit tokens, encased cents, and many others. In a narrow sense, trade tokens are "good for" tokens, issued by merchants. Generally, they have a merchant's name or initials, sometimes a town and state, and a value legend (such as "good for 5¢" or other denomination) somewhere on the token. Merchants that issued tokens included general stores, grocers, department stores, dairies, meat markets, drug stores, saloons, bars, taverns, barbers, coal mines,