Several theories of clarinet action have been developed, but have been based on rather fragmentary experimental data. The present paper offers such a description of the instrument as is essential to this particular study, and a consideration of the tone quality at the bell of the instrument in the three conventional registers. Special consideration is given to the harmonic structure of the tones and its variation throughout the pitch range. All pianissimo tones are extremely simple, consisting almost entirely of the fundamental. For this reason, the investigation of only the forte tones is necessary for the study of harmonic structure, since the mezzoforte tones represent a transitional stage. The harmonic structure simplifies in progressing from the lowest to the highest pitch with the extremes representing remarkable contrast. In the whole range of forty tones there are only five, blown forte, in which the percent of total energy in the fundamental exceeds fifty percent. The tone B4, which has the maximum energy, 88.2 percent, in the fundamental and no harmonics of comparable intensity, is the orchestral tuning note. The characteristic tone quality of the clarinet, often described as reedy or hollow, and most evident in the low register, appears to be due to the relatively strong odd numbered partial tones and to be determined mostly by the first eight partials.