Chapter 2 presents the book’s main argument about how party rules shape membership. Previous literature is split in its portrayal of party members – some scholars describe members as extremist ideologues, whereas others depict them as partisan loyalists. To reconcile these competing views, Chapter 2 develops a spatial model of membership in which members receive utility from government policy and party proximity, as well as features of party membership unrelated to ideology. The model demonstrates that party rules play a pivotal role in shaping a party’s overall membership level and distribution. The model predicts that decentralized parties attract more members than centralized parties, all else equal. However, decentralized parties’ members should be more ideologically extreme than their counterparts in centralized parties.