A discussion of the quantum Hall effect is presented which explicity describes the current and voltage contacts of a Hall sample. An explicit description of contacts is necessary whenever contacts are spaced closely compared to an equilibration length. A distinction is made between ideal contacts which feed or detect all states equally and contacts with internal reflection which feed or detect states unequally. The theory is applicable to highly non-uniform samples. A (normal) quantized Hall resistance is predicted only for samples with ideal contacts or for samples in which ideal contacts and contacts along the sample perimeter alternate. For two adjoining contacts there is at least one Hall resistance which deviates from the normal quantized Hall resistance. Surprisingly, experiments have shown that the equilibrium lengths in high mobility samples are macroscopic. Therefore, contacts play an important role also in determining the accuracy of the quantum Hall effect.