What describes a defect parallel to the grain on the surface of timber?

The choice of surface check as the correct answer is based on its specific definition in the context of timber defects. A surface check refers to additional separations or splits that occur in the surface layer of timber, running parallel to the grain direction. These checks are typically the result of natural drying processes or stress within the wood and can compromise the timber's structural integrity if they are significant.

In contrast, the other options represent different types of defects: blistering can occur when moisture builds up under the surface, shake is a defect that occurs inside the wood, often parallel to the growth rings, and split refers to a more severe and complete fracture through the timber, which typically goes across the grain rather than following it. Understanding these distinctions helps in correctly identifying and assessing defects in timber materials.