2 1/6 of the lower column diameters. Optical refinements To loosen up the mathematical strictness and to counteract distortions of human visual perception, a slight curvature of the whole building, hardly visible with the naked eye, was introduced. The ancient architects had realised that long horizontal lines tend to make the optical impression of sagging towards their centre. To prevent this effect, the horizontal lines of stylobate and/or entablature were raised by a few centimetres towards the middle of a building. This avoidance of mathematically straight lines also included the columns, which did not taper in a linear fashion, but were refined by a pronounced "swelling" (entasis) of the shaft. Additionally, columns were placed with a slight inclination towards the centre of the building. Curvature and entasis occur from the mid 6th century BCE onwards. The most consistent use of these principles is seen in the Classical Parthenon on the Athenian Acropolis. Its curvature affects all horizontal elements up to the , even the walls reflect it throughout their height. The inclination of its columns (which also have a clear entasis), is continued by architrave and triglyph frieze, the external walls of the also reflect it. Not one block of the building, not a single architrave or frieze element could be hewn as a simple rectilinear block. All architectural elements display slight variations from the right angle, individually calculated for each block. As a side effect, each preserved building block from the Parthenon, its columns, walls or entablature, can be assigned its exact position today. In spite of the immense extra effort entailed in this perfection, the Parthenon, including its sculptural decoration, was completed in the record time of sixteen years (447 to 431 BCE). Decoration Colouring Only three basic colours were used: white, blue and red, occasionally also black. The , columns, and architrave were mostly white. Only details, like the horizontally cut grooves at the bottom of Doric capitals (), or decorative elements of Doric architraves (e.g. taenia and guttae) might be painted in different colours. The frieze was clearly structured by use of colours. In a Doric triglyph frieze, blue triglyphs alternated with red metopes, the latter often serving as a background for individually painted sculptures. Reliefs, ornaments, and pedimental sculptures were executed with a wider variety of colours and nuances. Recessed or otherwise shaded elements, like mutules or triglyph slits could be painted black. The paint