What are patent families? What is the impact of adopting one definition or another? Are some definitions of patent families better suited than others for certain uses in statistical and economic analysis? The aim of this paper is to provide some answers to these questions, compare the methodologies and outcomes of the most commonly used patent family definitions and provide guidance on how to build families based on raw data from the EPO Worldwide Patent Statistics database (PATSTAT). One of our findings, based on a characterisation of family structures, is that extended patent families and other family definitions, such as equivalents and single-priority families, provide identical outcomes for about 75% of the families with earliest priority dates in the 1990s because they have quite simple structures. Differences across definitions only become apparent for the families with more complex structures, which represent 25% of the families of that period.