The tao tsang or canons (accepted texts) of all Taoist scriptures were divided into three tungs. The root meaning of the word tung is cave, but it also means "to communicate, to understand the mysteries." The term thus evokes the mysterious mountain grottoes, real or imaginary, harboring hidden revelations and dealings with the sacred powers.
道教的經典符咒方法，都收在「道藏」內，凡三洞四輔十二類，廣如浩瀚，深不可側，未窺門徑實難鑽研。