The concept of margin is at the very heart of legal Systems. The effect of internationalisation, or even globalisation of law, has been to introduce, as a consequence of the "national" margin of appreciation, both recognition of the diversity of legal Systems and possibility of a common law. This is why use of the comparative method is necessary. But the latter is insufficient, since it invites diversity without giving the key to a real legal pluralism, that is, ordered according to legal reasoning. Legal logic is helpful here, insofar as the concept of margin breaks with the traditional conception of the legal order as unified and hierarchical but not pluralistic. After a description of pluralism by juxtaposition, linked to the appea-rance of a national margin of appreciation (whether implied or express), the requirements for the logical validity of a real ordered pluralism (concept and functioning of the margin) will be considered.

Expression "law" does not mean the comparative law, but indicates an intellectual method consisting in comparing among them legal Systems or elements of these Systems (objects oflaw, relations between such objects). Therefore, two essential questions, which are inextricable, are the following ones : why compare, and how to proceed to the comparison ? The purpose of this article is to contribute the most simply possible, by means of generalizable examples for any comparative search, for the resolution of the second of these questions, by suggesting atfirst a reflection on the most elementary aspects of the operation of comparison (recognizing lhat objects are comparable, or to see to it that they are comparable), then by showing interactions between comparison and questioning, finally to widen the domain of the objects ofthe comparison (relations, Systems), which belong to a "universe" of which it is necessary to become aware of its limits.