Some eastern canoes having their greatest width, or beam, on the gunwales at midlength had finer level lines aft than forward, apparently to produce trim by the stern when afloat and manned. This made them steer well in rough water. Some northwestern canoes had their greatest beam abaft the midlength, giving them a long, sharp bow; the run was sometimes formed by sweeping up the bottom aft to a shallow stern, as well as by the double-ended form of the canoe. Despite a general similarity in the form of the ends, in some canoes the bow was marked by its greater height, in others, by the manner in which the bark was lapped at the seams, or by the manner of decoration. In a few with ends exactly alike the bow was indicated by the fitting of the thwarts such as, for example, by placing at the forward end a particular style of thwart, intended to hold the torch used in spearing fish at night, or to support a mast and sail. In examining the lines, or model, of a bark canoe, the limitations imposed upon the builder by the characteristics of bark must be considered. The degree of flexibility, the run of the grain, and the toughness and elasticity of the bark used all influenced the form of canoes. The marked chin in the ends of some canoes, for example, resulted from an effort to offset the tendency of birch bark to split when a row of stitches lay in the same line of grain. The curved chin profile allowed the stitching to cross a number of lines of grain. Sometimes this tendency was avoided by incorporating battens into the coarse stitching; this style of sewing was particularly useful in piecing out birch bark for width in a canoe, where the sewing had to be in line with the grain. The Indians also employed alternating short and long stitching in some form for the same purpose. Spruce bark, as used in canoes in the extreme North and Northwest, could be sewn in much the same manner as birch bark, but with due regard for the longitudinal grain of the spruce bark. The joining of two pieces of bark by root sewing or lacing, combined with the use of spruce gum to obtain watertightness, formed a seam that could be readily damaged by abrasion from launching the canoe, from pulling it ashore, or from grounding it accidentally. For this reason, seams below the waterline were kept at a minimum and were never placed along the longitudinal centerline of the bottom, where they would have formed a sharp apex to both the ~V~-shaped midsection and to the deadrise bottom form. Likewise, a seam was not used in forming the rocker of the bottom. Though seams had to be used to join the bark at bow and stern, the form of the canoe allowed the seams to be greatly strengthened and protected there.