Latvian language
The Middle dialect is spoken in central and Southwestern Latvia. Kursenieki language, which used to be spoken along Curonian Spit, is closely related to the varieties of middle dialect spoken in Courland. The dialect is divided into the Vidzeme variety, the Curonian variety and the Semigallian variety. The Vidzeme variety and the Semigallian variety are closer to each other than to the Curonian variety, which is more archaic than the other two. There are three intonations in the Middle dialect. In the Semigallian variety, "ŗ" is still used. Standard Latvian is based on the middle dialect.
Latvian language
According to some glottochronological speculations, the Eastern Baltic languages split from Western Baltic (or, perhaps, from the hypothetical proto-Baltic language) between 400 and 600. The differentiation between Lithuanian and Latvian started after 800, with a long period of being one language but different dialects. At a minimum, transitional dialects existed until the 14th century or 15th century, and perhaps as late as the 17th century.