by the national concepts being compared. Preparing an empirical investigation concerning adoption, the French researcher, for example, has to recognize the distinction between the national concepts 'adoption pleniere' and 'adoption simple in order to determine the respect in which he wants to compare different legal systems. This distinction must be made since the comparative concept of full adoption does not refer to foreign legal rules corresponding to the French 'adoption simple or limited adoption. Juxtaposing articles 356 and 364 of the French Civil Code may help the comparatist to find the intension for this distinction: 'Adoption confers on the child a filiation which substitutes for its original filiation, the adopted child ceases to belong to its family of blood,. and: 'An adopted child remains in his family of origin and conserves all its rights therein . (12) The first sentence refers to the effects of full adoption, the second one concerns limited adoption Different legal effects establish the distinction between full adoption and limited adoption within the French system. These national concepts have different intensions Choosing the intension of full adoption, the researcher will exclude those forms of adoption which do not terminate all legal ties between the child and his biological family. The two intensions of full and of limited adoption are immanent concepts applicable to foreign rules. Further, the comparability assumption has to be tested since specific effects connected to legal rules could be national characteristics without counterparts in the other legal system under investigation The absence of legislative definitions makes it difficult to choose an immanent concept in order to determine the respect in which the researcher can compare different legal systems. In a case of absence of these definitions, the formation of immanent concepts depends on the structure of the nceptual systems in which these concepts have been embedded. If these systems show almost the same structures, there is an indication that immanent concepts can be found. However immanent concepts are of no empirical use if the hierarchies of legal systems are too divergent conceptually. Comparing the positions of concepts within their respective hierarchies is important to determine the relationship of these concepts but similar positions do not guarantee any conceptual correspondence fixing the comparability of the rules of different legal systems a common characteristic cannot be found if branches of law do not show sufficient correspondence at higher and lower levels. E.g ' real property' in English law and ' biens immeubles in French law are branches of law missing a common characteristic at a higher. more general, level: while real property' looks to procedure, biens immeubles looks to substance (13) This fact impedes the comparatist in his search for a shared characteristic at a lower, more specific, level in the conceptual hierarchies of the respective legal systems. A common characteristic cannot be found for the English concept of 'chattel mortgage on the one hand, and the French concept of hypotheque mobiliere' on the other. The researcher is confronted with a gap when looking for a conceptual equivalent since the English 'chattel mortgage and the French hypotheque mobilier belong to different branches of law. The lack of a common characteristic at a higher, more general level makes the structure of the conceptual systems being compared too divergent. This fact implies that immanent concepts are of no empirical use in this case. Exceptionally, the researcher could prefer functional concepts According to Constantinesco, Zweigert and Kotz's functional concepts are too broad and ill-defined. Contrary to these concepts, immanent concepts are useful in fine-tuning theby the national concepts being compared. Preparing an empirical investigation concerning 'adoption', the French researcher, for example, has to recognize the distinction between the national concepts 'adoption plénière' and 'adoption simple' in order to determine the respect in which he wants to compare different legal systems. This distinction must be made since the comparative concept of full adoption does not refer to foreign legal rules corresponding to the French 'adoption simple' or limited adoption. Juxtaposing articles 356 and 364 of the French Civil Code may help the comparatist to find the intension for this distinction: 'Adoption confers on the child a filiation which substitutes for its original filiation; the adopted child ceases to belong to its family of blood, ...'; and: 'An adopted child remains in his family of origin and conserves all its rights therein, ...'.(12) The first sentence refers to the effects of full adoption, the second one concerns limited adoption. Different legal effects establish the distinction between full adoption and limited adoption within the French system. These national concepts have different intensions. Choosing the intension of full adoption, the researcher will exclude those forms of adoption which do not terminate all legal ties between the child and his biological family. The two intensions of full and of limited adoption are immanent concepts applicable to foreign rules. Further, the comparability assumption has to be tested since specific effects connected to legal rules could be national characteristics without counterparts in the other legal system under investigation. The absence of legislative definitions makes it difficult to choose an immanent concept in order to determine the respect in which the researcher can compare different legal systems. In a case of absence of these definitions, the formation of immanent concepts depends on the structure of the conceptual systems in which these concepts have been embedded. If these systems show almost the same structures, there is an indication that immanent concepts can be found. However, immanent concepts are of no empirical use if the hierarchies of legal systems are too divergent conceptually. Comparing the positions of concepts within their respective hierarchies is important to determine the relationship of these concepts but similar positions do not guarantee any conceptual correspondence fixing the comparability of the rules of different legal systems. A common characteristic cannot be found if branches of law do not show sufficient correspondence at higher and lower levels. E.g. 'real property' in English law and 'biens immeubles' in French law are branches of law missing a common characteristic at a higher, more general, level: while 'real property' looks to procedure, 'biens immeubles' looks to substance.(13) This fact impedes the comparatist in his search for a shared characteristic at a lower, more specific, level in the conceptual hierarchies of the respective legal systems. A common characteristic cannot be found for the English concept of 'chattel mortgage' on the one hand, and the French concept of 'hypothèque mobilière' on the other. The researcher is confronted with a gap when looking for a conceptual equivalent since the English 'chattel mortgage' and the French 'hypothèque mobilière' belong to different branches of law. The lack of a common characteristic at a higher, more general, level makes the structure of the conceptual systems being compared too divergent. This fact implies that immanent concepts are of no empirical use in this case. Exceptionally, the researcher could prefer functional concepts. According to Constantinesco, Zweigert and Kötz's functional concepts are too broad and ill-defined. Contrary to these concepts, immanent concepts are useful in fine-tuning the