The distance between the keys ranges from 18 to 20 mm (ISO 9241). The alphabetical section must be clearly separated from the function keys (ISO 9241). Dark letters on a clear background are easier to read (ISO 9241) and contribute to an increased productivity (Snyder, 1990). The middle row of keys cannot be elevated more than 30 mm above the surface of the desk (DIN 2137). The keyboard must be tiltable and separate from the screen (Council Directive 90/270/EEC).