The wave theory in physics serves to explain a number of natural phenomena, and in particular, light. In the 20th century an attempt was made to apply that theory in dialectology: the dialectal theory of a language related to the isoglosses passing through it is treated as central and peripheral. The central region is located where there are the least isoglosses. In this paper the wave theory is used to explain the intermediate state of development of the dialects referred to as mesolect by the author. Its appearance can be interpreted as a consequence of the diffraction of the linguo-flow wave. The dialectal code encounters a surmountable obstacle and is transformed taking elements from the literary code. Examples of the Tran dialect are provided.