Context: In recent years, UML-based domain-specific model languages (DSMLs) have become a popular option in model-driven development projects. However, making informed design decisions for such DSMLs involves a large number of non-trivial and inter-related options. These options concern the language-model specification, UML extension techniques, concrete-syntax language design, and modeling-tool support.Objective: In order to make the corresponding knowledge on design decisions reusable, proven design rationale from existing DSML projects must be collected, systematized, and documented using an agreed upon documentation format.Method: We applied a sequential multi-method approach to identify and to document reusable design decisions for UML-based DSMLs. The approach included a Web-based survey with 80 participants. Moreover, 80 DSML projects11Note that it is pure coincidence that there were 80 participants in the survey and that 80 DSML projects were reviewed., which have been identified through a prior systematic literature review, were analyzed in detail in order to identify reusable design decisions for such DSMLs.Results: We present insights on the current state of practice in documenting UML-based DSMLs (e.g., perceived barriers, documentation techniques, reuse potential) and a publicly available collection of reusable design decisions, including 35 decision options on different DSML development concerns (especially concerning the language model, concrete-syntax language design, and modeling tools). The reusable design decisions are documented using a structured documentation format (decision record).Conclusion: Our results are both, scientifically relevant (e.g. for design-space analyses or for creating classification schemas for further research on UML-based DSML development) and important for actual software engineering projects (e.g. by providing best-practice guidelines and pointers to common pitfalls).