One of the more significant public buildings of the Agora is the Tholos, a round structure, with six interior columns and a propylon at the east that was added in the 1st century BC. It was the headquarters of the 50 prytaneis who served as the executive committee of the Boule (Council) for an interval of 35 or 36 days, after which they were replaced by prytaneis from another tribe, so that by the end of the year representatives of all ten tribes had a turn in the administration. Those in office dined in the Tholos, with one-third (i.e. 17) of them spending all night in the building so that there were always responsible officials on hand. In the Tholos were housed the official weights and measures of the Athenian state. It was built atop a pre-existent building complex of the mid-6th century BC, which had a similar functional purpose, and went out of use around AD 400.