Koinai

As to the Italian situation, John Trumper’s now classical diagnosis (Trumper 1997) seems a noteworthy example. He distinguishes two types of diglossia, and defines them on the basis of the relationship that, within the repertoire, dialects and dialectal koinai have with regional varieties of Italian.

Source: wiktionary

The processes of standardization of the vernacular languages, which took place in such different ways in the various countries during the sixteenth century, first of all came up against the irregularities in orthography common in the various traditions, inherited from the age in which the circulation of vernacular writings of a practical nature was generally more limited to local areas, and the overwhelming prestige of Latin had left the vernaculars, even in their literary manifestations, in a state of semi-spontaneity or experimentalism which did not give them the strength to go beyond the dimension of the regional koinai.

Source: wiktionary

From the present point of view, it seems very likely that the resemblances among the urban dialects are the consequence of continous convergence and the mutual leveling of several regional koinai (see the summary in Miller, 1986).

Source: wiktionary

Also, it should be noted that the vernaculars sometimes underwent horizontal levelling (koineisation) before the onset of standardisation. Examples are the dialect koinai of southern, especially Andalusian Spanish which developed in the late Middle Ages (thirteenth to fifteenth century) before standardisation set in (Villena 1996).

Source: wiktionary