In the comparatively new collecting field of modern design, the simple seat has a totemic status. The tale of 20th-century design: the rise of new materials, the advances in engineering and mass production techniques, the roller-coastering aesthetics (austere to flamboyant, to whimsical and back again to functional), the development of ergonomics - all this is a story told in the evolution of one artefact: the chair. 
'Modern' has a different resonance depending on whether you are an art dealer, an auctioneer, or a furniture aficionado. In terms of flat art, 'modern' is actually rather old hat - contemporary being the latest thing. In the auctioneer's rather recherché lexicon, 'modernism' comes before World War II and the collecting category that includes post-war chairs comes under 'design'. In the more general parlance of the numerous furniture dealers and interiors emporia that sell vintage homewares, 'modern' tends to cover the instantly recognisable products of the 20th century whose design was geared towards mass production rather than construction by craftsmen.