Spirit houses were an important folk custom which were used to ensure balance with the natural and supernatural world. Astrology was also a vital part to understanding the natural and spiritual worlds and became an important cultural means to enforce social taboos and customs. Lan Xang was at the center of the overland trade routes in Southeast Asia. In the north and northwest the overland trade routes from Burma and Lanna passed through Lan Xang and Sipsong Panna ( Xishuangbanna ) toward Yunnan, where they would join with the Chinese Tea-Horse Road. Trade in Luang Prabang would flow down the Mekong to Vientiane where it could then be transported overland to the headwaters of the Nan and Chao Praya rivers, or overland by ox cart or elephant over the Khorat Plateau to Roi Et. In the east the Annamite Range formed a barrier, but the areas of Muang Phuan and Xam Neua were regular points of commerce with Vietnam. Trade from Thakhek and Champasak flowed down the Mekong to the Island of Khong, where the goods would then be portaged past Si Phan Don and the Khone Falls to join with the Mekong again and on to the Khmer in the south. Lao merchants ("lam") would travel to Lao Theung and Lao Sung areas to exchange cloth, iron and silver for forest products, which would be floated via streams on bamboo rafts until they met with larger rivers. The principle Lao agricultural crops were glutinous rice and forest timber. Both were labor-intensive and were difficult to transport using the overland routes. Subsistence farming of root crops, bananas, gourds, cucumbers, yams, water buffalo, chickens, pigs and other domesticated animals was indigenous within Lan Xang. Forest products were generally easier to transport and traded at a higher value. Elephants, ivory, benzoin resin (similar to Frankincense), lac (used in lacquer production), cardamom, beeswax, rhinoceros horn, along with porcupine quills and a variety of skins were commonly traded. Of particular importance was the deer skin trade, which was in high demand in China and Japan and would reach its way to market having gone through Ayutthayan trade posts. Lao craftsmanship in silk production, weaving, gold, and especially silver was in high demand. Villages would specialize in a particular craft or skill where they would manufacture tools, weapons, pottery, paper, jewelry, alcohol (lao-lao), elephant training or other unique trades.