" Neither personal income tax nor social security is harmonised within the EU. Social security systems are coordinated at EU level whereas personal income tax in cross-border situations is governed by respective double tax treaties. In most EU countries, personal income tax and social security contributions are relatively distinct payments. This article examines problems surrounding the interaction between personal income tax and social security contributions on a national and international level based on a case study of cross-border employment between the Czech Republic and Denmark. As the Czech and the Danish systems are designed very differently, the case study allows for clear illustration of the issue at-hand. The aim is to identify the elements influencing the impact of different coordination rules in personal income tax and social security contributions, illustrate and discuss the potential problems of such mismatches between the two payments. The impact on final payments differs, not only due to the different levels of coordination of the payments, but also due to the different designs of the two national systems. Thus, it would be very difficult to address all the scenarios with a one size fits all measure for all the EU Member States that would overcome the differences in this coordination."

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