Growth rings
How they effect lumber
In this example board we can see growth rings in a curved pattern. Everyone knows you can count the rings to determine the age of a tree but did you also know the growth rings also determine the quality of lumber.
A board cut from the center of a tree including the pith (the very most center of the tree) is very poor quality lumber and contains a lot of stress. This type of lumber is often used in building materials like 2x4s for home construction. When was the last time you thought about how straight the lumber was inside the walls of your home?
Why does wood move?
Growth rings in a board want to flatten out. As lumber dries either air drying or in a kiln the growth rings cause the once straight board to twist, or even crack.
Knowing this to get the best possible material woodworkers prefer to buy lumber in slab form and pick and choose the best quality material for the project they're working on.
This board has good growth rings
DEFECTS
Although this board has nice growth rings we can see a defect in the middle of the board. When the project doesn't call for a really wide board we have the option to remove that material.
With the center defect removed we end up with two quality boards. Defects like knots can happen anywhere in a board not just the center.