A ritually and historically important calendar used throughout Mesoamerica in which the solar calendar of 365 days ran in parallel with a sacred 260-day ritual calendar of named days. The calendar round is a 52-year cycle, since both calendars begin on the same day only once every 52 years. Coefficients for days and months were expressed by bar-and-dot numerals, a system that is first known in Monte Albán I and that became characteristic of the Classic Maya. The basic structure of the Mayan calendar is common to all calendars of Mesoamerica. To identify a date of the Calendar Round, they designated the day by its numeral and name, and added the name of the current month, indicating the number of its days that had elapsed by prefixing one of the numerals from 0 through 19. A date written in this way will occur once in every Calendar Round, at intervals of 52 years. It is the meshing of the two Maya calendars, the Tzolkin and the Haab.