The durability of wood increases in direct proportion to its density. When assessing the density of wood, it is always necessary to know the moisture value at which its mass and volume are measured. Usually the density of wood is given as the density of dry air, which is measured when the mass and volume of wood is at a humidity level of 15% (or 12%). Density is also often given as fresh dried density. In this case, the wood is measured when dry. The volume saturation point (about 30%) is measured at high humidity. The tree's age rings contain less light-coloured spring timber than dark summer timber. In a normal pine tree, the average of summer timber is 25%, while in spruce it is about 15%. A short gap between the rings does not mean that the wood is denser or more durable. For example, the annual growth of pines in Lapland includes almost exclusively the less common spring timber, although the interval between rings is very small. The durability of wood is mainly affected by the direction in which the weight is loaded compared to the grain. In the grain direction, the bending strength is directly proportional to the density of the wood. In solid, unblemished wood, the flexural strength is as great as the tensile strength. The tensile strength in the direction of the grain is usually 10-20 times greater than the strength perpendicular to the grain. The tensile strength also depends on the density of the wood: The tensile strength of spring timber in pine trees is only 1/6 of that of summer timber. The compressive strength of naturally dried wood is about half of the corresponding tensile strength. The shear strength of wood is 10-15% of the tensile strength in the grain direction. Shear strength is weakened by knots, imperfections and cracks in the wood. The flexibility and durability of wood increases as its density increases. The elasticity coefficient of wood in the direction of the grain can be up to a hundred times greater than the same parameter perpendicular to the grain. In the radius direction, the elasticity coefficient is almost twice as large as the same parameter in the tangential direction. Since the properties of wood are largely dependent on different factors, it is recommended to classify it according to its purpose. The classification distinguishes between two main inspection criteria: appearance and durability-technical properties of wood. When classifying the quality and durability of wood, 90% of the quality criteria are related to knots. Strength classification can be performed visually or mechanically.