Although it is common in the literature to distinguish between cohesion and discipline, these terms are rarely demarcated properly and prominent legislative scholars use them interchangeably. The delineation presented in this article attempts not only to distinguish between these two concepts but to divorce them, using two different approaches to party unity. The first is sociological, which emphasises norms and roles. The second is institutional, and it stresses strategic incentives and constraints. Herein lies the distinction between cohesion (agreement, or shared preferences) and discipline (the enforcement of obedience). This article argues that parties are more than shared preferences. Parties matter, even once preferences have been taken into account. If this is the case, then it is important to distinguish between how preferences can hold parties together, how parties can hold themselves together when preferences are not enough, and how to delineate between these two separate determinants of party unity.