The iconic London Underground map, which has been in use continuously since 1933, is in fact a diagram of the network. It shows relationships rather than distances to scale and uses only vertical, horizontal and diagonal lines, with different colours for each of the Tube lines.

The map has become a design classic, implicitly demonstrating the importance of simplicity, economy and utility – all key values promoted by Modernist design. Yet it was devised and produced by an engineering draughtsman, Harry Beck, after he had been made temporarily redundant by London Underground. Beck reasoned that, ‘If you're going underground, why do you need bother about geography? … Connections are the thing’. Although often assumed to be based on electrical circuit diagrams, his ‘geoschematic’ map was apparently modelled on sewer mapping.

Previous maps of the Underground railway system had followed conventional mapping principles. Beck’s radical simplification was at first viewed sceptically, but after a successful trial production of 500 copies in 1932, it was published in a first edition of 700,000 in 1933. Soon it had become a vital element in Frank Pick’s campaign to project London Transport as a modern, rational and efficient system.

Since then Beck’s map, for which he received only five guineas, has become the basis (often unacknowledged) for most urban transit maps and many other transport equivalents.