Allen, Debra J. (2012). Historical Dictionary of U.S. Diplomacy from the Revolution to Secession. Lanham, Md.: Scarecrow Press. p. 84. ISBN9780810878952. «Basically, because of diplomatic protocol, a receiving state would not dispatch a representative with a higher rank than it has received, so when the U.S. sent ministers, it also received ministers, not ambassadors. ... The U.S. adjusted its ranking system in 1893 and began to send and receive ambassadors.»
