Political parties normally compete in elections individually. Yet, sometimes they join forces and form pre-electoral alliances. This rather unusual strategy contains both costs and benefits. In this article we try to identify those costs and benefits by opening up the black box of internal party decision making in considering pre-electoral alliance formation. We start by assuming that parties of different electoral sizes could have different motives to face the voter as one electoral list. Through in-depth interviews at the local level in Flanders, we have studied pre-electoral alliance formation for the municipal elections in 2006. We find that the arguments of large parties mainly focus on becoming the leading formation and thus claiming the initiative in coalition formation. Small parties have more varied motives for forming or failing to form a pre-electoral alliance.