The screw on the left was handmade in the late 18th century. Note the flat spot on the shaft, the irregular threads, blunt tip and the off center slot. The screw in the center is machine made around 1830. It has sharp, even threads, a cylindrical shape, blunt end and the slot is still off center. The screw on the right is a modern gimlet screw, post 1848, with tapered shaft, even threads, pointed tip and centered slot. Photo courtesy Fred Taylor.