Dutch rules for full adoption can be compared to the Austrian rules governing ' limited adoption Such sentences do not give immediate practical advice; they say something about the legal rules of different systems. If the comparability hypothesis is verified, the Dutch rules are really comparable to the Austrian rules. A comparability assumption which cannot be falsified does not contain observable criteria. To find observable criteria for the comparability assumption, the researcher can begin by analysing legislative definitions in order to determine the respect in which he can compare different legal systems. The following definitions of the Dutch appartementsrecht' and of the GermanWohnungseigentum' will illustrate this seemingly simple preparation of comparative studies. The Dutch 'appartementsrecht'is defined as follows: 'An apartment right means a share in the property which is involved in the division and includes the right to the exclusive use of certain portions of the building which, as indicated by their layout, are intended to be used as separate units.(3)Section I of the German Condominium Act definesWohnungseigentum'differently Residential property is the separate ownership of an apartment in connection with the co-ownership share of the joint property, to which it belongs. (4) Since the right to the exclusive useis mentioned only by the Dutch legislator, this characteristic does not offer a common perspective which could be chosen as a starting point. By contrast, the legislative definitions allow the researcher to compare in the uniform perspective of 'co-ownership. The comparative concept of'co-ownership'is the unequivocal intension which may be part of a comparability assumption. If hypothesis is verified, it is possible to make a real distinction of differences and similarities tween the Dutch 'appartementsrecht' and the German'Wohnungseigentum Concomitantly, I do not consider equivocal intensions to be observable criteria since concepts which prove to have different meanings cause the misinterpretation of research results. These comparative concepts are without empirical use for the reason that they amount to mixing up differences and similarities By definition, comparative concepts that are not applicable in comparative studies, are not applicable either in areas in which comparative law functions as an aid. Correspondingly, I make a distinction between the direct and indirect applicability of comparative concepts: their usefulness in comparative law(direct)and their usefulness in areas of application outside comparative studies (indirect). In sections 2 and 3 the conditions under which comparative concepts are directly useful are discussed; in section 2 three different forms of comparative concepts are dealt with and section 3 examines their relative usefulness in comparative law. Section 4 discusses the Internet as an area of practical application in which comparative law merely serves as an aid 2. The formation of comparative concepts 2. 1 Extensional concepts This section contains a brief analysis of three forms of comparative concepts: extensional concepts, functional concepts and immanent concepts. These forms have to be distinguished before examining their relative usefulness in comparative law, the subject of section 3. First, the formation of extensional concepts will be made clearDutch rules for 'full adoption' can be compared to the Austrian rules governing 'limited adoption'. Such sentences do not give immediate practical advice; they say something about the legal rules of different systems. If the comparability hypothesis is verified, the Dutch rules are really comparable to the Austrian rules. A comparability assumption which cannot be falsified does not contain observable criteria. To find observable criteria for the comparability assumption, the researcher can begin by analysing legislative definitions in order to determine the respect in which he can compare different legal systems. The following definitions of the Dutch 'appartementsrecht' and of the German 'Wohnungseigentum' will illustrate this seemingly simple preparation of comparative studies. The Dutch 'appartementsrecht' is defined as follows: 'An apartment right means a share in the property which is involved in the division and includes the right to the exclusive use of certain portions of the building which, as indicated by their layout, are intended to be used as separate units'.(3) Section 1 of the German Condominium Act defines 'Wohnungseigentum' differently: 'Residential property is the separate ownership of an apartment in connection with the co-ownership share of the joint property, to which it belongs'.(4) Since 'the right to the exclusive use' is mentioned only by the Dutch legislator, this characteristic does not offer a common perspective which could be chosen as a starting point. By contrast, the legislative definitions allow the researcher to compare in the uniform perspective of 'co-ownership'. The comparative concept of 'co-ownership' is the unequivocal intension which may be part of a comparability assumption. If this hypothesis is verified, it is possible to make a real distinction of differences and similarities between the Dutch 'appartementsrecht' and the German 'Wohnungseigentum'. Concomitantly, I do not consider equivocal intensions to be observable criteria since concepts which prove to have different meanings cause the misinterpretation of research results. These comparative concepts are without empirical use for the reason that they amount to mixing up differences and similarities. By definition, comparative concepts that are not applicable in comparative studies, are not applicable either in areas in which comparative law functions as an aid. Correspondingly, I make a distinction between the direct and indirect applicability of comparative concepts: their usefulness in comparative law (direct) and their usefulness in areas of application outside comparative studies (indirect). In sections 2 and 3 the conditions under which comparative concepts are directly useful are discussed; in section 2 three different forms of comparative concepts are dealt with and section 3 examines their relative usefulness in comparative law. Section 4 discusses the Internet as an area of practical application in which comparative law merely serves as an aid. 2. The formation of comparative concepts 2.1 Extensional concepts This section contains a brief analysis of three forms of comparative concepts: extensional concepts, functional concepts and immanent concepts. These forms have to be distinguished before examining their relative usefulness in comparative law, the subject of section 3. First, the formation of extensional concepts will be made clear