There is no definitive date when the various Italian variants of Latin—including varieties that contributed to modern Standard Italian—began to be distinct enough from Latin to be considered separate languages. One criterion for determining that two language variants are to be considered separate languages rather than variants of a single language is that they have evolved so that they are no longer mutually intelligible; this diagnostic is effective if mutual intelligibility is minimal or absent (e.g. in Romance, Romanian and Portuguese), but it fails in cases such as Spanish-Portuguese or Spanish-Italian, as native speakers of either pairing can understand each other well if they choose to do so. Nevertheless, on the basis of accumulated differences in morphology, syntax, phonology, and to some extent lexicon, it is not difficult to identify that for the Romance varieties of Italy, the first extant written evidence of languages that can no longer be considered Latin comes from the ninth and tenth centuries C.E. These written sources demonstrate certain vernacular characteristics and sometimes explicitly mention the use of the vernacular in Italy. Full literary manifestations of the vernacular began to surface around the 13th century in the form of various religious texts and poetry.Although these are the first written records of Italian varieties separate from Latin, the spoken language had likely diverged long before the first written records appear, since those who were literate generally wrote in Latin even if they spoke other Romance varieties in person.