grain is visible on the end of a barrel stave at the bevelled edge, which is also known as the chime. Since staves are split along the radial plane, it can be seen across the width of the stave end. Each light and then dark alternation represents a one-year growth ring for the oak tree. The average width of each annual growth ring is called the grain by coopers and barrel users.
Every year, the tree forms a new growth ring in the cambium layer under the bark. Seen under a microscope, a ring can be broken down as a succession of spring wood (or early wood) and summer wood (late wood). Vessels in spring wood are more numerous and wider than in summer wood. Summer wood therefore has a higher density of fibres and parenchyma and fewer vessels.