From the study of subharmonic series found in the violin and related instruments, it is evident that the quality and loudness of the instruments depend on the pitch and variety of the subharmonic series, and on the characteristics of the wood and the fashioning of the top and back plates. Details are given on the nature of subharmonic series and their presence as observed in several methods of testing. The single air tone, heretofore reported, is shown by more detailed tests to consist frequently of two or more closely related peaks. Observations made in a series of experiments carried on during the construction of 35 violins, violas, and cellos indicate a method of controlling tone quality and volume of the completed instrument by means of the tap tones and the acoustic properties of the free plates. Experimental equipment which makes these findings possible is described in detail. With this concept of subharmonic series and the techniques described, a large field for experimental evaluation of string instruments may well be opened up.