The OECD Model Tax Convention developed out of the League of Nations Models which were strongly influenced by the treaty practice between the mainland European countries. For that reason, a common law reader coming to the OECD Model for the first time finds it full of unfamiliar expressions. At least some of these difficulties are caused by the use of literal translations into English of expressions that would not be used in that sense in English, but the differences between common law and civil law are more important. The purpose of this article is twofold: to trace the derivation of the terms used in the OECD Model (including, as terms for this purpose, the categories of income and their definition) to their original state or states, and the converse - the extent to which the Model has influenced the internal law of states. The former may shed some light on the meaning of civil law expressions unfamiliar to common law readers and, to a far smaller extent, on the meaning of common law expressions unfamiliar to civil law readers. The article does not make any claim to be comprehensive as the source of many of the expressions may be lost in the mists of history or may have originated in states not represented by the authors. The authors' impression is that the Model has had less influence on internal law than might have been expected, perhaps so that states retain their treaty negotiating position. In conclusion, the authors have found virtually no concept in any of the Models that cannot be found in an earlier treaty. The Models have in turn influenced later treaties.