Something that is occasionally confusing to those interested in vacuum tubes of the 1920s is the old style of numbering. When RCA began marketing tubes, they were considered part of their stock of standard devices. Tubes therefore carried a designation consisting of two letters and then the tube's actual number. The two letters were not arbritary; they gave a clue as to the socket required by the tube. Thus:
"UV" indicated a base of four short pins (such as the UV-200A detector triode);
"UX" indicated a base of four long pins, two larger in diameter (such as the UX-171A power amplifier triode);
"UY" indicated a base of five long pins (such as the UY-224A tetrode, and the UY-227 general purpose triode)
As more tubes were added to the roster, and the advent of the six- and seven-pin bases, the system began to break down. By 1932, the new RETMA system of designating a tube had gone into wide use. On the older tubes, all but the last two digits of the tube number were dropped: for example, the UY-224A became the 24A.