The Chinese tan is an altar where ancient rulers used to offer sacrifices to Heaven or the gods, and architecturally it refers to a special type of terrace-like building. Several ancient tan in Beijing, mostly dating from the Ming (1368-1644) and Qing (1644-1911) dynasties, have remained in name if not in fact.

A number of Beijing's parks, for instance, still have the word tan in their names:

Tiantan (Temple of Heaven)

Yuetan (Altar to the Moon)

Ditan (Altar to the Earth)

Ritan (Altar to the Sun) , and

Xiannongtan (Altar to the God of Agriculture)

All five were first built in the Ming Dynasty for the worship of such fods as indicated in their names .On the grounds of what have become parks stood the altars of worship, either round terraces of three tiers (as in the Temple of Heaven) or square ones of one or two tiers.

The most celebrated altar in China is Huanqiutan (or the Circular Mound Altar ) in Beijing's Temple of Heaven ,religiously the most important temple construction .Built in 1530 under the Ming ,the all marble terrace is five meters high and consists of three tiers ,respectively 30,40 and 70 meters across. The terrace circular ,in keeping with the ancient Chinese belief that Heaven was round.

The number of the stones used for the surfaces of the terrace and on the steps and the number of the balusters for each tier are all nine (the highest masculine figure*) or its multiples . The terrace is devoid of any other structure and it was to this bare altar that the Ming or Qing Emperor came to offer sacrifices to Heaven . In a ceremony called “open-air rite”, obeisance was made to Heaven, unobstructed by anything overhead.

*See article entitled “The number 'Nine' and Imperial Buildings”