The study of the Lunar Series at Piedras Negras suggests that the Maya followed the same scheme of counting of lunar months from the date of the erection of Stela 30 (534 CE) to the time of the dedication of Stela 12 (795 CE). However, during the Late Classic Period (600-900 CE), some of its rulers initiated new calculation methods when they acceded to the throne. These methods consisted of modifying the starting day of the month or of changing month numbers. The paper shows that despite these changes, the Maya kept the intercalation rule intact. Examination of the Lunar Series suggests that the lunar count at Piedras Negras may be derived from the structure of the much later Xultun Lunar Table (early 9th century C.E.).El estudio de las Ser...