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This paper is devoted to O-type Carcinotron tubes, which are backward wave oscillators particularly suitable in applications requiring a tube with a very wide electronic tuning range and moderate power output. A survey of backward-wave operation is given. Theoretical results on the starting conditions are shown to compare favorably with experimental values. The effects of reflections are discussed and are shown to be very small only when the delay line has properly matched terminals. Approximate expressions for efficiency and frequencypushing are derived. Design and performance data are given for a series of tubes now in production, each of which covers about one octave in frequency, providing total frequency coverage from 1,000 to 12,000 mc. The maximum beam voltage of any of the tubes in the series is 1,500 volts. The rf power output of the series is 100 to 1,000 milliwatts. Data on delay lines, gun structures and focusing are given, and results of tests on 100 tubes are tabulated.