In the 19th century, when road travel was slow and treacherous and railroads had not yet been established, canal travel provided a dependable way to move passengers and cargo, helping link Pennsylvania's major cities with its rural communities. Canal boats were built from several kinds of wood chosen for specific purposes: white oak, a dense, strong, durable hardwood, was especially suited to the boat's skeleton, including the keel, stern posts, and frame, and was also used for planking; yellow pine, a dense, pitchy, rot-resistant softwood, was used for planking, deck beams, and ceiling; and white pine was used for decking because it shrank and swelled very little, helping seams remain tight and water resistant. The frames, like ribs joined to the backbone or keel, determined the shape of the hull. Boat builders rough-cut the frames with adzes, axes, and saws, held timbers in place with large clamps until they were nailed together, and added the side planking after the stern post and frames were set. Long, narrow boards lined the frames inside and outside to form the boat's skin, and through careful calculations builders cut the planks almost exactly to shape before using steam to fit them snugly to the hull's curves. Hard wooden pegs were driven through drill holes to fasten the planking to the frame, and planes smoothed the planking and decking. Planking alone did not keep out water, so caulking followed: hemp was jammed into the spaces between planks, tarred hemp called oakum was pushed into place with irons and mallets, hot molten pitch was brushed over the packed seams to make the boat watertight, and a fresh coat of paint was then applied.