The form or shape of fruit trees can be manipulated by pruning and training. Shaping and promoting a particular tree form is done to establish the plant in a particular situation under certain environmental conditions, to increase fruit yield, and to enhance fruit quality. For example, pruning a tree to a pyramid shape enables trees to be planted closer together.[citation needed] An open bowl or cup form helps sunlight penetrate the canopy, thus encouraging a high fruit yield whilst keeping the tree short and accessible for harvesting.[citation needed] Other shapes such as cordons, espaliers and fans offer opportunities for growing trees two dimensionally against walls or fences, or they can be trained to function as barriers.