A system can often be best represented by a diagram showing the constituent parts and their interconnections, often referred to as a reliability block diagram (RBD). Such a system can usually be described as having an input and an output, and a route through the system connecting the input to the output may be called a path. There are two basic types of system structure, series and parallel. A series system works only if all components work. A parallel system works provided at least one of the components works. A kind of decomposition is possible into groups of components that can fail or function together, in the sense that their failure or functioning can directly affect the functioning of the system. The most important such groups are ties and cuts, which are generalizations of the concepts of series and parallel systems.