Scholars from all around the world have acknowledged the importance of distinguishing between formal and informal aspects of the functioning of political systems in general (e.g. Helmke & Levitsky, 2004; Lauth, 2000; Tsai, 2006, Grzymala-Busse, 2010), and judiciaries in particular (e.g. Popova, 2012; Kosař et al. 2024; Dressel et al., 2017; Llanos et al., 2016). To understand how a judicial branch is organized and how it performs its tasks, it is necessary to look beyond the formal framework setting up the ‘rules of the game’. However, even if we identify informal judicial institutions and practices operating within a judiciary, our understanding is not entirely complete as informal judicial institutions and their co-existence with a formal framework can produce a variety of effects. In this article, we aim to (a) map informal judicial institutions and practices in 6 European jurisdictions (Hungary, Poland, Italy, Germany, Slovakia and Czechia) and (b) demonstrate how informal institutions interact with formal ones within both judicial decision-making and governance areas. Our analysis builds on a unique dataset of approx. 90 semi-structured elite interviews with judges, politicians, journalists, legal experts and practitioners accompanied by the respective laws and regulations on the organization of judicial branches. In illustrating the relationship between formal and informal judicial institutions, we draw on Helmke and Levitski’s (2004) typology of informal institutions in which they distinguish between complementary, substitutive, accommodating, and competing informal institutions. We demonstrate the formal-informal relationships by using four selected case studies originating from the European jurisdictions under study.