It is useful to divide a cloud into two regions: core and margin. Three parameters used to define a core are compared: buoyancy (B), relative humidity (RH), and vertical velocity (W). Using theoretical arguments and simulations, we show that during most of a cloud's lifetime, the cores are subsets of one another: B_core ⊆ RH_core ⊆ W_core. Moreover, the core–shell cloud model applies to all core definitions. Our findings can serve as a benchmark in the partition the core and margin.