Why is the 会 in 会计 pronounced kuài? The character 会 has two different pronunciations; usually it is huì but in 会计 it is pronounced kuài. It seems that the latter pronunciation is exclusively used in the "accounting" meaning.

How did this one character take on two different pronunciations and meanings? Was it a merger of two different characters?
In Old Chinese, it is generally thought that some words followed regular morphological alternations (which are preserved in a few places in MSM, but &quot;frozen&quot;, i.e., no longer productive). For instance:

Verb/Noun
处 chu3 &quot;to dwell&quot; / chu4 &quot;a place&quot;
数 shu3 &quot;to count&quot; / shu4 &quot;a number&quot;
知 zhi1 &quot;to know&quot; / (also 智) zhi4 &quot;knowledge&quot;

Similarly:

Active/Inactive Verb
好 hao4 &quot;to like&quot; / hao3 &quot;to be good&quot;
系 ji4 &quot;to tie&quot; / xi4 &quot;to be connected&quot;
会 kuai4 &quot;to bring together&quot; / hui4 &quot;to come together&quot;

Phonetic reconstructions for these alternations generally follow a regular pattern (e.g., voiced/voiceless, 去声/other tone), although specific reconstructions differ.
会 is a somewhat odd example, in that the hui4 reading has displaced the kuai4 reading in almost all places except proper nouns and 会计 = accounting. Branner gives the example from Confucius:

君子以文会友，以友辅仁

The customary reading of 会 in this passage is hui4, but etymologically, it really should be kuai4.
My references are taken from Branner's excellent essay, &quot;On Early Chinese Morphology and its
Intellectual History&quot;
EDIT:
The Middle Chinese readings corresponding to &quot;hui4&quot; and &quot;kuai4&quot; are &quot;hwajH&quot; and &quot;kwajH&quot; in Baxter's notation; the initials being 匣 and 見. Based on this, one would expect the MSM pronunciation of the latter to be &quot;gui4&quot; rather than &quot;kuai4&quot;. The common Japanese On reading of かい &quot;kai&quot; (attested earlier as くわい &quot;kwai&quot;) is actually a kan-on reading. According to wikipedia, the k- initial in a kan-on reading could correspond to either a 匣 or a 見 MC initial. In any event, the &quot;kai&quot; reading is used in both the 'meeting' and 'accounting' meanings, regardless of which MC reading it corresponds to. Some speculate (e.g., Victor Mair, p31) that the MSM &quot;kuai4&quot; may have been influenced by the Japanese &quot;kai&quot; (i.e., that the etymologically expected &quot;gui4&quot; was displaced by &quot;kuai4&quot; when 会计 was re-borrowed from Japanese to mean &quot;accounting&quot;). It is not the case, however, that the hui4/kuai4 distinction is an artifact of Japanese borrowing--a meaning of &quot;settling accounts&quot; is attested dating back well before the Japanese neologism for accounting (see halfway down random blogpost quoting from dictionary).