Divination boards were fairly common in China. They consisted of a round disc, of various diameters, usually of wood either lacquered or varnished. In the centre was a magnetic compass, and around it, covering the entire surface, was a circular azimuth chart. It had the characters for the cardinal points of the compass, the Twelve Terrestrial Branches, the Ten Celestial Stems, and other calendrical characters at various intersections of the radial and concentric lines on the chart. On the back of the disc were painted other similar characters in a rectangular chart. The charts were used to measure the degrees of azimuth of certain natural phenomena that were thought to influence the course of events. In this way auspicious and inauspicious days were established, and the proper rituals were observed to celebrate fortuitous occurrences, to ward off calamities, or to appease vengeful spirits, whichever seemed to be necessary in the astrological circumstances prevailing at the time. In a modified form, these charts appear around the compasses of Chinese sundials. Other, earlier forms of divination boards appeared on the dial surface of ancient sundials and on TLV mirrors (mirrors bearing marks resembling these letters) of the Han period (206 BC - AD 220). The specific use of the earlier boards is even less clear than that of the later models.