Chile's congressional elections are governed by a binomial system that, for the most part, rewards the two largest representations equally, often regardless of their relative popular support. Parties are thus forced to form wide coalitions and, historically, the two largest coalitions (Concertación and Alianza) split most of the seats. Only if the leading coalition ticket out-polls the second place coalition by a margin of more than 2-to-1 does the winning coalition gain both seats, which tends to lock the legislature in a roughly 50–50 split.