the word ‘Sonder’ of which one translation is ‘special’, linked to the word ‘Klasse’ which translates as ‘class’, or category’. The name ‘3=6’ started out as an advertising slogan, but by the time of the 1955 face lift (for the 1956 model year), the name was to the fore, and the car was advertised as the ‘Large 3=6’ (Großer 3=6) differentiating it from the earlier version which already carried the script ‘3=6’ ahead of the door on its left side. The point of the advertising slogan was to highlight an equivalence between the car's two stroke three cylinder engine and a four stroke six cylinder engine. The underlying logic was that with the two-stroke cycle there is engine power produced by a combustion within each cylinder for every rotation of the crankshaft: with the four-stroke cycle there is power produced by a combustion within each cylinder only for each alternate rotation of the crankshaft. Thus it was asserted that the two-stroke engine was working twice as hard per rotation of the engine. In terms of torque the two-stroke system does indeed appear to have conferred substantial benefits when compared to a four-stroke engine of similar size, but in terms of bhp much of the theoretical energy gain in terms of power output seems to have been dissipated as additional heat which in turn required a larger energy consuming cooling fan, all of which made the arrangement rather noisy when placed just ahead of the driver and front-seat passenger. The name F91 was the factory project number of the car. ‘F’ stood for ‘Frontantrieb’ (Frontwheel drive). The F91 was an evolution from the DKW F9 which had been a prototype presented in 1938, planned for production at Auto Union's Zwickau plant from 1940. By 1950 the F9 itself had been made production ready and was being produced as the IFA F9 in Zwickau, so that name was in practice not available to ‘old’ Auto Union's western successor. The DKW F91 was replaced by the F93 followed by the F94, their names also taken from factory project numbers. Because the other names have proved increasingly unfathomable, the names F91, F93 and F94 are the ones commonly used retrospectively. It was perhaps in recognition that any perceived marketing advantages available from the unconventional namings had been exhausted, that from 1958 the car was sold simply as the DKW 900 (in the USA only),