It is not only the written but also the customary law which requires interpretation; but the method is quite different in the latter. Here it becomes necessary to gather from the various rules of customary law the principle, and to test by this principle the true nature of any particular usage. Above all in the case of the customary practice of the courts will the interpreter have to consider what reason lies at the bottom of any decision. Perhaps two or three reasons are given in an opinion, and one of these is based on the customary practice. The question will then arise whether the custom is the real reason which led the court to decide as it did, or whether it was nothing but a more or less weighty consideration which merely influenced but did not determine the decision. This is the way in which the opinions of the courts are interpreted in England and America, in order to discover whether some rule is authoritatively enunciated. At the same time, the question is then considered whether such rule is applicable only to the particular state of facts found in the case at bar, or is of so general a nature as to apply also, even though one or the other of the details of fact were lacking.

8 See my "Handbuch des Patentrechts," pp. 396 seq.