quality:corruption, structural:summarization
Semantic motivation

The ki-/vi- class historically consisted of two separate genders, artefacts (Bantu class 7/8, utensils and hand tools mostly) and diminutives (Bantu class 12/13), which were conflated at a stage ancestral to Swahili. Examples of the former are kisu "knife", kiti "chair" (from mti "tree, wood"), chombo "vessel" (a contraction of ki-ombo). Examples of the latter are kitoto "infant", from mtoto "child"; kitawi "frond", from tawi "branch"; and chumba (ki-umba) "room", from nyumba "house". It is the diminutive sense that has been furthest extended. An extension common to diminutives in many languages is approximation and resemblance (having a 'little bit' of some characteristic, like -y or -ish in English). For example, there is kijani "green", from jani "leaf" (compare English 'leafy'), kichaka "bush" from chaka "clump", and kivuli "shadow" from uvuli "shade". A 'little bit' of a verb would be an instance of an action, and such instantiations (usually not very active ones) are found: kifo "death", from the verb -fa "to die"; kiota "nest" from -ota "to brood"; chakula "food" from kula "to eat"; kivuko "a ford, a pass" from -vuka "to cross"; and kilimia "the Pleiades ", from -limia "to farm with", from its role in guiding planting. A resemblance, or being a bit like something, implies marginal status in a category, so things that are marginal examples of their class may take the ki-/vi- prefixes. One example is chura (ki-ura) "frog", which is only half terrestrial and therefore is marginal as an animal. This extension may account for disabilities as well: kilema "a cripple", kipofu "a blind person", kiziwi "a deaf person". Finally, diminutives often denote contempt, and contempt is sometimes expressed against things that are dangerous. This might be the historical explanation for kifaru " rhinoceros ", kingugwa " spotted hyena ", and kiboko " hippopotamus " (perhaps originally meaning "stubby legs").

Another class with broad semantic extension is the m-/mi- class (Bantu classes 3/4). This is often called the 'tree' class, because mti, miti "tree(s)" is the prototypical example. However, it seems to cover vital entities neither human nor typical animals: trees and other plants, such as mwitu 'forest' and mtama 'millet' (and from there, things made from plants, like mkeka 'mat'); supernatural and natural forces, such as mwezi 'moon', mlima 'mountain', mto 'river'; active things, such as moto 'fire', including active body parts (moyo 'heart', mkono 'hand, arm'); and human groups, which are vital but not themselves human, such as mji 'village', and, by analogy, mzinga 'beehive/cannon'. From the central idea of tree, which is thin, tall, and spreading, comes an extension to other long or extended things or parts of things, such as mwavuli 'umbrella', moshi 'smoke', msumari 'nail'; and from activity there even come active instantiations of verbs, such as mfuo "metal forging", from -fua "to forge", or mlio "a sound", from -lia "to make a sound". Words may be connected to their class by more than one metaphor. For example, mkono is an active body part, and mto is an active natural force, but they are also both long and thin. Things with a trajectory, such as mpaka 'border' and mwendo 'journey', are classified with long thin things, as in many other languages with noun classes. This may be further extended to anything dealing with time, such as mwaka 'year' and perhaps mshahara 'wages'. Animals exceptional in some way and so not easily fitting in the other classes may be placed in this class.

The other classes have foundations that may at first seem similarly counterintuitive. In short,