The most common type of harmony happens to vowels, maybe because of the loosey-goosey, spectrum-rather-than-point nature of vowel pronunciation, and essentially any feature of vowel articulation can be subject to harmony (including one that doesn't really exist except to explain harmony - more in a bit). Let me give a few examples.
Everyone's favorite example of vowel harmony is Turkish because it's so neat and easy to understand. Turkish has eight vowels:
kedi "cat" - kediler "cats" (front)
ayı "bear" - ayılar "bears" (back)
High vowels also participate in rounding harmony, so the ordinal number suffix has four different forms:
iki "two" - ikinci "second" (front unround)
üç "three" - üçüncü "third" (front round)
altı "six" - altıncı "sixth" (back unround)
dokuz "nine" - dokuzuncu "ninth" (back round)
Neat! These examples of Turkish do a pretty good job of illustrating that each item in a harmonic category usually has a counterpart in other harmonic categories, like how a's front counterpart is e here. A common theme of harmonic systems is that affixes will contain a sound underspecified for category, which will appear as the corresponding sound in whatever category the rest of the word follows (like how the plural suffix -l(vowel)r contains a high unround vowel whose harmonic frontness category is determined by earlier vowels).
Frontness and rounding are pretty common subjects for harmony. Another classic example is Finnish, which also has a harmony system based on frontness:
Another very common feature for vowel harmony to work on is "advanced tongue root" (ATR). I'd contend that ATR harmony systems are actually the most common type, but they seem to get less attention because they appear in less familiar languages, and because it's ultimately a harder concept to nail down. This is the one I mentioned that only exists to explain vowel harmony - ATR isn't an absolute feature of a vowel like frontness or rounding, but a spookier one that vowels can only have relative to other vowels. It seems to me like ATR is a combination of frontness, centrality, and height. +ATR vowels are fronter and/or higher and/or less central than their -ATR counterparts.
For instance, here's the vowel system for Igbo:
For the purposes of harmony, /i̙ a u̙ o̙/ are -ATR, contrasting with +ATR /i e u o/.
In terms of morphology, ATR systems work in pretty much the same way as what we've already seen, with affixes aligning with the harmony category of previous vowels. In fact, harmony and affixation seem to go hand-in-hand - languages with more affixing are more likely to exhibit vowel harmony. That makes sense - without affixes, harmony doesn't have anything to operate on. It's also thought that in highly agglutinating languages, where words can get huge, harmony helps to signal word boundaries - if the harmonic category changes, that's a good sign a new word just started.
There are loads of other types of harmony, including some that get consonants involved! Vowels and consonants can both be nasal, giving ample opportunity for harmonization, and it turns out that post-velar consonants get along really well with -ATR vowels, too. For more on that I recommend checking out the Rose and Walker paper.
|Doesn't Rose and Walker sound like a hipster restaurant? I think I've got Parker and Otis on the brain.|
Rose and Walker
Languages with vowel harmony: