Ports existed because of trade. As often as possible ships carried the goods exchanged. Transport over water was superior because of the simple physical relation between the lower levels of friction between a solid and a liquid than between a solid and a solid. From the High Middle Ages to the early seventeenth century ships went through one of the greatest transformations in the history of their use by human beings. There were significant improvements in the vehicles of transportation over water. The changes were not just in the design of the ships but also in how they were manned, how captains and crews navigated and in the ways people shifted their cargoes in ports. The developments took place against a background of long-term swings in the volume of trade. While at the end of the thirteenth century traffic among ports had risen to the highest level of the Middle Ages there was a significant fall through the second half of the fourteenth century. Recovery was slow, though by the late sixteenth century it appears that commerce was at or beyond the levels of 1300. In 1600 the character of trade was considerably different with new commodities and new routes incorporated into maritime commerce.