Political parties that wish to exercise executive power in parliamentary democracies are typically forced to enter some form of coalition. Parties can either form a pre-electoral coalition prior to election or they can compete independently and form a government coalition afterwards. While there is a vast literature on government coalitions, little is known about pre-electoral coalitions. A systematic analysis of these coalitions using a new dataset constructed by the author and presented here contains information on all potential pre-electoral coalition dyads in twenty industrialized parliamentary democracies from 1946 to 1998. Pre-electoral coalitions are more likely to form between ideologically compatible parties. They are also more likely to form when the expected coalition size is large (but not too large) and the potential coalition partners are similar in size. Finally, they are more likely to form if the party system is ideologically polarized and the electoral rules are disproportional.