The goal of this chapter is to develop a taxonomy of decentralisation measures and how they are related to each other. In addition to introducing a common language for the different strands of literature, this taxonomy is instrumental for studying the outcomes of decentralisation. Using cross-state data from the United States, we show that aggregating distinct dimensions of fiscal decentralisation into a single indicator inevitably leads to a loss of information in the form of lower explanatory power. We conclude that the distinct aspects of decentralisation should enter regression analyses separately, in the most flexible functional form possible. In particular, we find that revenue autonomy is virtually orthogonal to the subnational share of revenues and expenditures, suggesting that it carries additional information. In this chapter we show also how the conventional measures of decentralisation can be modified to account for the differing dependence on external grants.