Although the Tibetan calendar was devised primarily around farming, it still incorporates some elements of the ancient Indian system of division of time and the lunar calendar of the Han, the largest ethnic group in China. The Tibetan calendar year is divided into 12 months, each of which numbers 29 or 30 days. Every three years, an intercalary month is added to adjust to farming activities. The Tibetans have borrowed the ancient Chinese notions of yin and yang and the combination of 10 celestial stems and 12 terrestrial branches to create a cycle of 60 years. The 10 celestial stems consist of five yin and five yang, named after the five basic elements: wood, fire, earth, metal and water. The 12 terrestrial branches are represented by the rat, ox, tiger, hare, dragon, snake, horse, sheep, monkey, rooster, dog and pig respectively. These names are similar to those in the Han lunar calendar.