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Although arrays of Josephson junctions are today's primary reference standards of voltage, they are used by only 30 to 40 national metrology institutes around the world and perhaps a comparable number of primary metrology laboratories. Nearly all traceable voltage measurements are linked to Josephson standards by chains containing electronic DC voltage standards referenced to Zener diodes. These typically take the form of small, rather easily transportable instruments that provide outputs of 10 V and 1.018 V. The specified stability over a 1-year period of the 10 V outputs of the best Zener standards is quite good, of the order of one part in 106 of the nominal output voltage, and, by modelling temporal drift and correcting for environmental influences, output voltages can be predictable, over periods of several weeks or more, to within a few parts in 108. However, given that the demonstrated reproducibility of 10 V Josephson standards is of the order of a few parts in 1010 of the nominal output, an enormous increase in measurement uncertainty occurs in going from Josephson standards to Zeners. The various effects that limit the reproducibility of Zener standards, including drift, pressure, temperature and humidity effects and, ultimately, 1/f noise, are reviewed.