Built doorways are some of the most insistently rectangular elements in the world. \'V'hy? \'Ve can all think of some non-rectangular doorways: gaps in the stones of a burial chamber; arched ones in churches; circular ones in Chinese gardens; ones with a corner missing squeezed into a tight loft-space .... But, generally speaking, doorways tend towards the rectangular: with vertical sides Gambs); a horizontal threshold; and a horizontal or arched head.