A microwave spectrograph designed for measurements of linewidth is described. The width spectrograph consists of the same general major components as a spectrograph for frequency measurements, but differs considerably in the requirements of the components and the assembly thereof since many factors which have virtually no effect on the frequency of a spectral line could influence the width significantly. Methods are given to generate a stable and pure frequency-modulated microwave signal which has a constant power output over a frequency range of 10 Mc. Great precaution must be made in assembling the waveguide components to reduce reflection in the absorption cell. For each crystal detector one must select the optimum load resistance (generally ranging from 3 kΩ to 1 MΩ) to insure square-law response since deviation from this behavior may introduce an error as large as 4%. A discussion of the general procedure of linewidth measurements which includes modulation and length corrections is given.