The BIPM specifies twenty prefixes for the International System of Units (SI). All ten of the fractional prefixes are lowercase. However, only seven of the multiplicative prefixes are uppercase, the exceptions being, da, h, and k, that is, the first three.

The fact that these prefixes seem to be older than the larger ones is a possible reason. The conflict between k (kilo) and K (kelvin) could also be a reason, but it seems unlikely to be the case as m (milli) and m (metro) are the same letter.

Is there any reliable source about why those three (and only those three) prefixes are lowercase?

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    $\begingroup$ The fact that these prefixes seem to be older than the larger ones is a possible reason. Did you consider that all the lowercase prefixes are smaller than all the upper case ones? $\endgroup$ – G. Smith Aug 18 at 4:58
  • $\begingroup$ @Farcher but the suffix is capital k "K" and that's what we're talking about here. $\endgroup$ – Carl Witthoft Aug 21 at 12:05

In 1795 lower case abbreviations were proposed for the prefixes myria, kilo, hecto, deca, deci, centi, milli: m, k, h, d, d, c, m. They were rarely used until after 1840, when the temporary mesures usuelles were replaced by the original unit names of the metric system. By then capitals were often, but not always, used for the multiples (myria, kilo, hecto, deca: M, K, H, D) and lower case for the submultiples (deci, centi, milli: d, c, m).

In 1879, a few years after the introduction of the CGS system, lower case abbreviations were internationally adopted for all common prefixes, because a combination of upper- and lower-case would be too susceptible for errors, and it would curb the ease of writing. Myria was excluded from the recommendation, because it would otherwise get the same abbreviation as milli. Initially hecto and deka/deca were also excluded, hecto because h was objectionable for Italy (the prefix is "etto" in Italian), and deka/deca because dk was unacceptable for France. A few decades later h and dk/da were included in lowercase. 1

So, at the end of the 19th century, all common prefixes were lower case for the convenience of ordinary people, commerce and trade. Capitalized prefixes were reserved for scientists who needed higher prefixes.

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