The capillary system in wood is divided into two major categories: large capillary system and micro capillary system.

There are two broad categories of capillary systems in wood, namely large capillary systems and microcapillary systems. The moisture in the wood is present in these capillary systems.

The large capillary system consisting of cell cavities has little binding force to moisture and no binding force, and moisture can freely evaporate from the cross section of the large capillary system. Therefore, the water present in the large capillary system is called free water. The increase or decrease of free water can only affect the weight, preservation and burning ability of wood without affecting the nature of wood.

A microcapillary system consisting of interconnected cell walls with varying degrees of binding force to moisture. To evaporate moisture in the microcapillary system into the air, the humidity of the air must be reduced to a certain extent; or under heating conditions. Accelerate the movement of moisture to overcome the binding force of the microcapillary and evaporate into the air. At the same time, the microcapillary system not only evaporates water into the air under certain conditions, but also absorbs moisture from the air. Therefore, the water present in the microcapillary system is called sorption water. The increase or decrease of sorption of water not only causes the wood to expand and contract, but also affects other physical and mechanical properties of the wood.

In addition, there is a compound water in wood, which is present in the chemical composition of wood. The mathematics of the combined water is very small, and it only makes sense when it is chemically processed.

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