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Perhaps the first self-consciously designed type system was Adrian Frutiger's Univers family, which comprised 23 individual typefaces in a range of weights, widths, and slants. Type systems are groupings of typefaces where the individual typefaces are meant to complement one another, either in conception or in appearance. The early type systems were almost always designed around the sans serif typefaces popular in the Swiss typography of the 1950s, and the goal was to show a smoothly modulated procession from thin, narrow typefaces through thick, wide typefaces, with all the faces in between clearly being members of the same family. Perhaps the clearest portrayal of this system is the poster Frutiger designed to announce Univers's arrival, itself a paragon of Swiss-style design.
At the end of the 20th century, type systems as an organizing principle grew both in popularity and in conception, as designers created type super-families that encompassed sans serif, slab serif, and serif versions of the same underlying idea. Perhaps the most fully-fleshed version of this is Lucas de Groot's Thesis.