[en] Harmonization is the process of making information and estimates comparable across administrative borders. The degree to which harmonization succeeds depends on many factors, including the conciseness of the definitions, the availability and quality of data, and the methods used toconvert an estimate according to a local definition to an estimate according to the reference definition. Harmonization requires the availability and use of common reference definitions and methods for converting from estimates based on national definitions to estimates based on reference definitions. This article focuses on conversion methods, which are characterized as « bridges » because they can be seen as a means of crossing from islands of local definitions to the mainland of a reference definition. A structured approach is proposed for constructing bridges of three kinds : reductive, neutral, and expansive bridges. A hierarchical decision tree is presented to guide users and to summarize the propositions and case examples with different types of bridges to illustrate the concepts. Although the article addresses harmonization of forest information, the results are relevant for harmonizing a broad variety of area statistics.