A school principal's workload is recognised as being heavy, with an imbalance between demands and resources. This paper contributes to the development of collective leadership. The principalship constellations of six schools in Sweden were studied with the aim of strengthening the current knowledge about structures and experiences of shared principalship. The empirical basis is qualitative data from interviews with principals and vice-principals. The analytical focus was on how the sharing structures were organised and how the shared principalship was experienced. The results point to a considerable variation in the organisational structures of shared principalship. Despite the type of model, form and constellation, the principals and vice-principals voiced a striking sense of relief in not feeling alone in their duties, as problems and troubles became manageable. An intensified interaction level in the principalship constellation created opportunities to develop competence. Theoretically, this study broadens the invited leadership concept to include horizontal invitations across unit boundaries between principals in different units within the same school. The knowledge contribution of this study is useful in discussing the legal possibilities for shared principalship, which may be especially relevant in times when the Swedish school system is being criticised for not delivering good student outcomes.