[Square Hole Size]: 1/4 inch / 6.4 mm, 5/16 inch / 8 mm, 3/8 inch / 9.5 mm, 1/2 inch / 12.7 mm. Refers to the edge length of the square hole drilled by the square drill bit. [Application]: This mortising chisel is ideal for drilling square holes in wood with a mortising machine or mortising attachment. Suitable for use in the woodworking, construction, installation and decoration industries. [Features]: Reduce the labor intensity of construction workers, accelerate the construction speed. [Material]: High strength and hardness, made of high quality material, wear resistance and easy to use. [Note]: Other dimensions are shown in the picture. Please avoid 1-2mm errors due to manual measurement.