This chapter considers respects in which the explicit recognition of the mesh principle may modify and perhaps notably improve comparative linguistics. The explicit recognition of mesh principle in linguistic relationships can be of tremendous help in determining and demonstrating the placement of isolated languages and linguistic groups. The methodologic lesson to be drawn from the considerations is that one should never confine himself entirely to the internal evidence of linguistic grouping under study if there is any possibility of obtaining additional light from other, somewhat more distantly related languages. The proof of common origin and the phonologic, and lexical reconstruction of the older periods of language are operations which depend on an adequate body of comparative material. To prove the systematic correspondence of a given phoneme of one language with a given phoneme of another language, it is necessary to have enough sets of convincing cognates showing the equivalence to eliminate the possibility of chance coincidence.