The main difficulty in working with the U.S. length units is in adding or subtracting dimensions and especially when converting them into metric, both in making and in using drawings for general construction and machine technology. These are the problems and the areas Multilane is intended for as an...http://www.google.co.uk/patents/US5272653?utm_source=gb-gplus-sharePatent US5272653 - Multilane keyboard for inch calculators
