The OASIS Universal Business Language (UBL) effort has several interesting goals and constraints that must be taken into account in the structuring of the UBL Library schemas. This paper discusses some of the major rules developed for the design of the schemas: UBL's connection to the UN/CEFACT–ebXML Core Component Technical Specification, its choice of options for element and datatype definitions, and its solution for reusable code lists. These rules are presented in the hope that they may be found useful to others embarking on an effort to define a standard XML vocabulary.