The transition from Latin to Asturian was slow and gradual; for a long time they co-existed in a diglossic relationship, first in the Kingdom of Asturias and later in that of Asturias and Leon. During the 12th, 13th and part of the 14th centuries Astur-Leonese was used in the kingdom's official documents, with many examples of agreements, donations, wills and commercial contracts from that period onwards. Although there are no extant literary works written in Asturian from this period, some books (such as the "Llibru d'Alexandre" and the 1155 "Fueru d'Avilés") had Asturian sources.