After the launch of the Audi 100 of 1968, the Audi 80/Fox (which formed the basis for the 1973 Volkswagen Passat) followed in 1972 and the Audi 50 (later rebadged as the Volkswagen Polo) in 1974. The Audi 50 was a seminal design because it was the first incarnation of the Golf/Polo concept, one that led to a hugely successful world car. Ultimately, the Audi 80 and 100 (progenitors of the A4 and A6, respectively) became the company's biggest sellers, whilst little investment was made in the fading NSU range; the Prinz models were dropped in 1973 whilst the fatally flawed NSU Ro80 went out of production in 1977, spelling the effective end of the NSU brand. Production of the Audi 100 had been steadily moved from Ingolstadt to Neckarsulm as the 1970s had progressed, and by the appearance of the second generation C2 version in 1976, all production was now at the former NSU plant. Neckarsulm from that point onward would produce Audi's higher-end models.