Directors on boards of nonprofit organizations can have additional director positions in other nonprofit organizations. When several of these interlocking directors exist for a group of nonprofit organizations, a board network is formed. We investigate to what extent similarity between organizations in terms of size, funding structure and operational activities relates to the presence of shared board members between organizations. For a network of 610 organizations we test and confirm that board networks are not formed at random, but that similarity of organizational characteristics explains interlocking behaviour, and that clusters of similar organizations exist within the overall nonprofit sector. Given this observation we propose three areas for further research. In particular we discuss opportunities regarding potential effects of network clustering, the causal direction of the relationship found, and the complementarity of the board network to other social networks in the nonprofit sector.