The vast majority of elections in parliamentary systems result in minority situations. During cabinet formation, parties have three options: building a winning coalition, a genuine substantive minority cabinet without support, or a formal minority with institutionalised long-term support partnerships. Even though the use of permanent support partners has increased substantially, there is still comparatively little knowledge about the circumstances under which parties choose to enter such formalised support partnerships instead of winning coalitions. This article aims to close this gap by analysing how the party system, the institutional configuration, as well as the bargaining environment influence which cabinet type forms. The dataset includes 469 cabinets from 27 Eastern and Western European countries between 1970 and 2019. The hypotheses are tested with the help of multinomial model estimations. While only few of the traditional explanations can explain the formation of formal minority cabinets, the results show that there is a time-trend towards more formalisation.