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In operation of a frequency-modulated station it is necessary to monitor the frequency deviation accurately so that the signal remains within the allocated band. This is important from the standpoint of adjacent-channel interference and the distortion produced in a receiver when the frequency devsation exceeds the limits of the detector. This paper presents an analytical discussion and computed charts of the spectra produced by frequency-modulated waves. A monitor which plots the amplitude and frequency distribution of the spectrum on the screen of a cathode-ray oscilloscope is described and calibration of the monitor is discussed. Photographs taken from the screen of the monitor are presented and compared with computed patterns.