Parliamentarianism is an important political institution that determines how executive power is controlled, and a key part of many representative democracies. In this paper, I explicitly define parliamentarianism as a system where the government is dependent on the tolerance of the parliament for its survival, but not on the tolerance of any body other than parliament. I argue that these are the necessary and sufficient conditions to consider a regime parliamentary. This definition opens up for an investigation of parliamentary systems in time and space, and I test its usefulness on the case of Belgium 1830-2014. I show that the Belgian parliamentary regime has been characterized by breaks, where the monarch has neutralized the parliamentary principle and at some points in time undermined the relationship between parliament and government.In contrast to prior politico-historical comparative research, I further show that, although parliamentarianism was inscribed in the Belgian constitution in 1831, the current uninterrupted Belgian parliamentary regime does not go further back in time than the 1950s. The investigation shows that my definition yields new insights and a viable empirical strategy. I include a suggestion for why Belgian parliamentarianism only existed for short periods before the second World War, and why Belgium is now experiencing its longest parliamentary spell.