A diode usually is thought of as a device that lets current flow through it in only one direction; however Zener diodes are made to permit current to also flow in the reverse direction if the voltage is larger than the rated breakdown or "Zener voltage". The breakdown voltage can be controlled quite accurately in the doping process. Tolerances to within 0.05% are available, though the most widely used tolerances are 5% and 10%. The effect was discovered by the American physicist Clarence Melvin Zener.