The production of veneer, whether by slicing or peeling, subjects the wood to high stresses and distortions. Even though the knives of a lathe have low sharpness angles of about 20°, the wood is still deflected at a sharp angle at the knife tip (Figure 1). This causes the veneer to split at regular intervals on the knife side of the veneer. These splits are called tension or lathe checks (see Figure 2). Their presence changes the properties of the surface, particularly in terms of permeability. Consequently, the face without the tension checks is called the "tight side" and the face with the checks is the "loose side". In general, the thicker the veneer, the greater the chance of tension checks being formed. This is because the minimum radius of curvature for a thick veneer is larger than that for a thin veneer or in other words, thin veneer is more pliable than thick veneer. If the checking is very bad, then the veneer can break. To help minimise tension checks, the wood is kept saturated and often heated to ensure that it is as pliable as possible. A compression force applied just ahead of the knife tip with a nose bar can also reduce tension checking. When laying-up plywood, the veneers placed on the outer faces should be the orientated so that their tight faces are exposed. If this is not done, then there is a risk of excessive checking on the surface with time as the veneer moves with changing atmospheric conditions. Next time you see a plywood, especially one with thick veneers, examine the edges and you may see the tension checks described above.