Also known as a two-centre arch. It is best described as the inclination of two segmental arches, from opposite springing points, which meet with a point at the top and must key with a joint and never with a key brick. There are three forms of the arch based on the triangle that sets the span related to the rise. Equilateral: constructed on an equilateral triangle with centres at the ends of the span. Lancet or Acute: the rise is greater than the span and places the two centres outside the span. Drop or Obtuse: the rise is less than the span and places the two centres within the span.