Flexibility markets are a promising tool to make better use of existing distribution grids. We analyse four pioneering projects implementing flexibility markets: Piclo Flex, Enera, GOPACS, and NODES. Based on a literature review, we develop a six-question framework. We find that all of the considered flexibility markets are operated by a third party. All projects also engage with multiple DSOs to become the standardized platform provider. Differences among the projects are found in the extent to which the flexibility markets are integrated into other existing markets, the use of reservation payments, the use of standardized products, and the way TSO-DSO cooperation is done. The answers to these questions vary for the projects because of different visions, use cases, or project maturity. Our case study analysis of four pioneering projects enriches the taxonomy and shows that practice is moving faster than the conceptual debate around flexibility markets.