In times when the biggest possible cargo loads over land were limited by horse-wagons, most cargo was transported by ship. Because most waters in the Netherlands are very shallow, a special ship-type was developed; the so-called "platbodem". Types such as the "tjalk" were already built in wood in the seventeenth century. As these ships have little draught, the wind will set them off course very easily. The "swords" on the side prevent this (they have the same function as a keel) and can be pulled up when the water is real shallow. Compared to other ship-types, the cargo-capacity of "platbodems" is very large.