2.1 Assimilation / Dissimilation
Assimilation means as much as "to become more alike", whereas dissimilation means "to become less alike". Both describe how sounds in a word influence each other and thus change each other. Especially assimilation is very common in all languages and an important process in sound changes.
2.1.1 Assimiliation
When there are two sounds near (not necessarily neighboring) each other (which is said to be happening from time to time ;)), they represent two different positions of tongue and manner of airflow. By putting for example the
places of articulation closer together, pronounciation effort can be reduced. Using less energy for pronouncing sounds is generally an underlying principle of sound change! So, because we are lazy even in our unconcious speech behaviour, we tend to "slur" sounds into each other - which is called assimilation.
We can now distinguish between several different kinds of assimilation.
First of all, there is
total assimilation where one sound becomes essentially another. As an example Trask states Latin /nokte/ becoming Italian /notte/ (double t in IPA states
gemination, I don't know if that is a general convention) and thereby totally assimilating the [k] into the geminated [t].
Second, as a contrast to this, there is
partial assimilation. Actually, most assimilations are partial. As an example you could consider the Japanese word <konbanwa> which is pronounced /kom.baŋ.wa/ - this is of course no diachronic (i.e historical) change! - You can see, that the written <n> turns out to be pronounced as [m]. This is due to the [ b] that follows: Both [m] and [ b] have the same point of articulation - bilabial. The alveolar sound [n], which was to be expected from the spelling is partially assimilated to be more alike to the [ b].
Another distinction possible is between
contact and
distant assimilation. We talk about contact assimilation when the two sounds
influenced are neighboring each other - just as in the examples above. Distant assimilation, as opposed to that, is when two sounds not neighboring
each other influence each other. There is an example in Trask from Latin: There is a preform of "five" which is supposed to be *[peŋkʷe] which has
then changed to *[kʷeŋkʷe] resulting in the word
quinque (after another vowel change). In that example, the [kʷ] in the middle of the word had influenced the [p] in the beginning to totally assimilate.
Regarding the sequence in which the sounds occur in the word (e.g. first [p] then [kʷ]), we can distinguish another category:
anticipatory or
regressive assimilation and
perseverative or
progressive assimilation. The example from Latin above was a regressive assimilation, because the assimilated sound was
before the one causing the assimilation, so you could look at it like the [kʷ] in the middle of the word "reaches forward" through the word - which is
not in the sense of the reading progression - that's why it's called regressive. These notions may very well be confused, even in Linguistic literature. Trask advises to use "anticipatory" and "perseverative" respectively.
Another possibility we haven't considered yet is that the two sounds may change both as well. This is called
mutual assimilation.
Okay, that was a lot! But let's break it down to the basics: We have
two dimensions of assimilation now. One being the
distance and one being the
direction. The distance can either be
contact, with the sounds next to each other or
distant; the direction can either be
anticipatory (from right to left),
perseveratory (from left to right) or
mutual (in both directions at the same time).
2.1.2 Dissimilation
Dissimilation is the opposed process to assimilation. That means, two sounds become more distinct from each other. This seems to contradict the principle I stated before - by dissimilating two sounds, the effort to pronounce them
rises. So when does dissimilation occur? First of all, dissimilation is by far more seldom than assimilation. It occurs when there is a repetion of sounds in a word. Trask mentions the Italian word <colonello> and the Spanish equivalent <coronelo>, where the [l] is dissimilated to [r], probably to avoid a repetion. Intrestingly, English uses the Italian orthography and the Spanish-like dissimilated pronounciation (cp. <colonel> -> /ˈkɜː.nl̩/ or /ˈkɜ˞ː.nl̩/).
Also, when two sounds of the same
manner of articulation occur in succession, they may as well be dissimilated.