It can be considered both as a forerunner of, and a challenge to, the rather better known concept of Max Weber’s: the ideal type (in German Idealtyp).
[1] Tönnies drew a sharp line between the realm of conceptualization (of sociological terms, including ‘normal types’) and the realm of reality (of social action).
Following Tönnies, reality (the second realm) cannot be explained without concepts, which belong to the first realm, or else you will fail because you try to define x by something derived from x. Tönnies’ Normaltyp was thus a conceptual tool created on a logical basis,[2] an almost mathematical concept always open to subsequent refinement from a confrontation with the empirical evidence.
[3] The contrast with Weber’s ‘ideal type’ came from the latter’s ‘accentuation’ of certain elements of a real social process, which is under sociological (or historical) scrutiny - “the one-sided accentuation of one or more points of view ... of a great many diffuse, discrete, more or less present and occasionally absent concrete individual phenomena”, as Weber himself put it.
Nevertheless, Weber’s term survived in the sociological community, since his Idealtyp helped to understand social forces, and for him sociology had both to explain and to understand things – a daring combination, but successful in the eyes of many sociologists.