The tun is the third unit of the Maya Long Count, equal to eighteen winals, or 360 days. It is at this level that the Long Count departs once from strict base-twenty counting: eighteen winals rather than twenty are gathered into a tun, so that the tun of 360 days approximates the length of the solar year, though it falls five days short of the true 365. The tun is a fundamental unit of the count, and Maya rulers marked the completion of tuns and their multiples with monuments and ceremonies. Above the tun the scheme returns to multiplying by twenty. A Long Count date names the tally of each unit since the mythic starting point. These are the features of the Maya Long Count, the linear calendar carved on Classic-period stelae to record the passage of these day-counts.
A Tun is eighteen Winal, 360 days, the third place in the Long Count and the place where the system departs from pure base-20 counting to approximate the solar year. Twenty Tun make one Katun, 7,200 days, so the vigesimal logic resumes above the Tun place; the 360-day Tun is the single irregular step in an otherwise consistent base-20 positional system, comparable in principle, though unrelated in origin, to the way the Babylonian sexagesimal system mixed bases for practical calendrical reasons. Because a Tun of 360 days runs roughly five and a quarter days short of the true solar year, Long Count dates and the 365-day Haab drift relative to each other over long spans in a documented, predictable way that Maya astronomer-priests tracked and that modern correlation studies use as one check on proposed correlation constants between the Long Count and the Gregorian calendar.