Gold certificates, along with all other U.S. currency, were made in two sizes—a larger size from 1865 to 1928, and a smaller size from 1928 to 1934. The backs of all large-sized notes and also the small-sized notes of the Series of 1934 were orange, resulting in the nickname “yellow boys” or "goldbacks". The backs of the Series of 1928 bills were green, and identical to the corresponding denomination of the more familiar Federal Reserve Notes, including the usual buildings on the $10 through $100 designs and the less-known abstract designs of denominations $500 and up. With the 1934 issue, the promise to pay was amended with the phrase "as authorized by law", as redemption was now restricted to only certain entities. The phrase "in gold coin" was changed to "in gold" as the physical amount of gold represented would vary with changes in the government price. Both large and small size gold certificates feature a gold treasury seal on the obverse, just as U.S. Notes feature a red seal, silver certificates (except World War II Hawaii and North Africa notes) a blue seal, and Federal Reserve Notes a green seal.