like all tube numbering systems, there are many inconsistencies between theory and practice. for example, there is no assigned letter code for cathode-ray tubes. some unusual types received rather mundane sounding designations, based solely on electrode count, because there was no better place to put them. for example, the 2f21 is not an actual hexode, but a pattern generating monoscope tube. some very exotic types received generic designators, even when there was a more appropriate designator available. for example, the 2h21 "phasitron" phase modulator tube used in early fm broadcast transmitters was assigned an "h" (octode) designator, when it would have been a perfect candidate for the otherwise unused "t" category for deflection controlled tubes.