In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells. The three principal shapes associated with epithelial cells are squamous, cuboidal, and columnar. Squamous epithelium has cells that are wider than their height (flat and scale - like). This is found as the lining of the mouth, oesophagus, and including blood vessels, and in the alveoli of the lungs, the Bowman's capsule and the Loop of Henle. Cuboidal epithelium has cells whose height and width are approximately the same (cube shaped). Columnar epithelium has cells taller than they are wide (column - shaped). Columnar epithelium can be further classified into ciliated columnar epithelium and glandular columnar epithelium. On the basis of structural modification of the cells, simple epithelium is further divided into three types - squamous, cuboidal and columnar epithelium.