We have seen several models of physical systems and errors in the examples of the previous sections. Most physical systems under consideration for QIP consist of particles or degrees of freedom that are spatially localized, a feature reflected in the error models that are usually investigated. Because we also expect the physically realized qubits to be localized, the standard error models deal with quantum errors that act independently on different qubits. Logically realized qubits, such as those implemented by subsystems different from the physically obvious ones, may have more complicated residual error behaviors.