Figure 3-12 illustrates several important characteristics of physical DFDs. First, we observe that each bubble contains a number as well as a title. Including a number in each bubble makes it easier to reference it later. This also assists designers in the decomposition tasks discussed shortly. Second, we notice that a physical DFD includes the same inputs and outputs as its predecessor context diagram in Figure 3-11—i.e., the context DFD and the physical DFD are balanced. This balancing is important because unbalanced DFDs are inconsistent and therefore probably contain errors. Third, we find that all the bubbles in the physical DFD contain the names of system entities (i.e., the titles of employees). These titles should correspond to the titles in an official organization chart.