The sound chamber of the qin is constructed with two boards of wood, typically of differing wood types. The slightly rounded top board (soundboard) is usually made of "tong" wood 『桐』, the Chinese parasol tree, or Chinese paulownia. The bottom board is made of "zi mu" 『梓木』 catalpa ("Catalpa ovata") or, more recently, "nan mu" 『楠木』 camphor wood ("Machilus nanmu"). The wood must be well aged, that is, the sap and moisture must be removed (of the top board wood). If sap remains then the sound will not be clear and, as the moisture evaporates, the wood will warp and crack. Some makers use old or ancient wood to construct qins because most of the sap and moisture has been removed naturally by time (old "shan mu" 『杉木』, Chinese Cunninghamia or Japanese Cryptomeria, is often used for creating modern qins). Some go to lengths to obtain extremely ancient wood, such as that from Han dynasty tomb structures or coffins. Although such wood is very dry, it is not necessarily the best since it may be infected with wood worm or be of a bad quality or type. Many modern qins made out of new "tong" wood (such as those made by Zeng Chengwei) can be better than the quality of antique qins.