Also called defects, they can be divided into three categories:
 　① Natural defects. Such as knots, oblique textures and defects caused by growth stress or natural damage. Knots are the branches wrapped in the xylem when a tree grows. Twill texture of logs is often called twist texture, and sawn wood is called twill texture.
 　　② Defects of biological harm. It mainly includes decay, discoloration and moth eaten< br> ③ Defects caused by drying and machining. Such as dry cracking, warping, sawing wound, etc. Defects reduce the use value of wood.
 　　In order to reasonably use wood, the types, sizes and quantities of allowable defects of wood are usually limited according to the requirements of different uses, and the wood is classified for use. Decayed and moth eaten wood is not allowed to be used in the structure, so the defects affecting the structural strength are mainly knots, twills and cracks.