When using an analogue voltmeter, there is no theoretical limit to how small a displacement the pointer can experience. There is, of course, a practical limit: for example, with the voltmeter shown above, a change from 2·45v to 2·46v might just be visible but any smaller change would be totally undetectable.

With a voltmeter having a digital display, the changes in the reading only occur in discrete "steps". A variable which can only change in this way is said to be quantised and for this reason, the precision of a digital instrument is said to be limited by its quantisation error which is simply the smallest amount by which the display of the instrument can change. So, when recording a result measured by a digital instrument the indeterminacy is ±1 in the least significant digit (in other words, the last decimal place) of the display. The reading of the meter in the diagram is therefore 2·4 ±0·1v.

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