were introduced. However, the limitations of HF transmission such as excessive error rates due to multipath distortion and the nature of ionospheric propagation kept many users at 60 and 66 speed. Most audio recordings in existence today are of teleprinters operating at 60 words per minute, and mostly of the Teletype Model 15. Another measure of the speed of a teletypewriter was in total "operations per minute (OPM)". For example, 60 speed was usually 368 OPM, 66 speed was 404 OPM, 75 speed was 460 OPM, and 100 speed was 600 OPM. Western Union Telexes were usually set at 390 OPM, with 7.0 total bits instead of the customary 7.42 bits. Both wire-service and private teleprinters had bells to signal important incoming messages and could ring 24/7 while the power was turned on. For example, ringing 4 bells on UPI wire-service machines meant an "Urgent" message; 5 bells was a "Bulletin"; and 10 bells was a FLASH, used only for very important news, such as the assassination of John F. Kennedy. The teleprinter circuit was often linked to a 5-bit paper tape punch (or "reperforator") and reader, allowing messages received to be resent on another circuit. Complex military and commercial communications networks were built using this technology. Message centers had rows of teleprinters and large racks for paper tapes awaiting transmission. Skilled operators could read the priority code from the hole pattern and might even feed a "FLASH PRIORITY" tape into a reader while it was still coming out of the punch. Routine traffic often had to wait hours for relay. Many teleprinters had built-in paper tape readers and punches, allowing messages to be saved in machine-readable form and edited off-line. Communication by radio, known as radioteletype or RTTY (pronounced ritty), was also common, especially among military users. Ships, command posts (mobile, stationary, and even airborne) and logistics units took advantage of the ability of operators to send reliable and accurate information with a minimum of training. Amateur radio operators continue to use this mode of communication today, though most use computer-interface sound generators, rather than legacy hardware teleprinter equipment. Numerous modes are in use within the "ham radio" community, from the original ITA2 format to more modern, faster modes, which include error-checking of characters. Control characters A typewriter or electromechanical printer can print characters on paper, and execute operations such as move the carriage back to the left margin