Archaeologists in China have unearthed a mysterious set of rectangular wooden pieces linked to an ancient astronomical calendar. The artifacts were discovered inside an exceptionally well-preserved 2,000-year-old tomb in the southwest of the country, Live Science reports. Each of the 23 wooden slips is about an inch (2.5 centimeters) wide and 4 inches (10 cm) long and displays a Chinese character related to the Tiangan Dizhi, or "Ten Heavenly Stems and 12 Earthly Branches"—a traditional Chinese astronomical calendar established during the Shang dynasty, which ruled from about 1600 B.C. to about 1045 B.C. Circular perforations at the edges of each slip suggest they were once tied together. This is the first time such objects have been found in an ancient tomb, although the practice of writing characters on strips of wood or bamboo was common in China before the invention of paper.