In the construction of a spline joint, how should the grain be oriented?

• A. with the grain
• B. diagonal to the joint
• C. along the edge of the stock
• D. across the joint

Answer: (D)

Grain orientation matters because wood fibers carry load best along their length. In a spline joint, you want the spline to run across the joint so its fibers cross the glue line. This cross-grain orientation gives the glue a broader, more reliable fiber path to resist forces trying to slide the pieces apart and to resist twisting or pulling apart along the joint. It also helps prevent tear-out and improves alignment by distributing the load across more fibers. If the grain runs with the joint or along the edge, the joint becomes more prone to splitting or slipping because the fibers aren’t working across the joint to carry the load as effectively.