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This is the fifth part in a series of notes that will help teachers understand what SI is and how to use it in a common-sense way. This part discusses units not to be used with SI.

Avoid all non-SI units that have an SI equivalent except the ones specifically accepted with SI and listed in Table IV, Part III of this series.1 If a unit appears to be metric, but is not listed in Tables I–IV, Part III, it should not be used with SI. Many of these units have been introduced into the U.S. mix of units in earlier attempts to “go metric,” before the more systematic SI was developed.

The units discussed in (a) and (b) below are listed in SI because they are used in older literature and are found in legal documents; however, their use loses the advantage of the coherence of SI...

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