Wood contains growth rings, which are formed by seasonal variations in wood growth. The inner part of the growth ring, formed in spring and summer, is known as early wood. The outer part, formed in autumn and winter, is known as late wood. There are also some cells, known as rays, which grow in the radial direction. These also provide transport and storage of nutrients.

Wood is a highly anisotropic material and it is therefore important to distinguish the orientation of wood with respect to the direction in which it grows. The directions are shown below. Axial is the direction along the long axis of the wood, radial is the direction along a radius of the circular trunk cross-section, and tangential is the direction that is at right angles to the radial direction.

Due to the anisotropic nature of the wood, its properties are highly dependent on the direction that it is cut. Most timber is cut in a tangential-axial manner or in a radial-axial manner. As a result of the orientation of the fibres within the wood, the strength in the axial direction can be 25 to 30 times greater than the strength in the radial or tangential directions.