Most of the forms of lexeme formation that we’ve looked at so far have involved adding something to a base (or combining bases).1 Some languages, however, have means of lexeme formation that involve changing the quality of an internal vowel or consonant of a base, root, or stem; sometimes this internal change occurs alone, and sometimes in conjunction with affixation of some sort. Such processes are called internal stem change or apophony.

Vowel changes: ablaut and umlaut

 Example (7) gives some words where internal vowels change:

In Manchu, in forming the female equivalent of a male noun, back vowels become front vowels. In Muskogee, verb stems have five forms each of which can be used in a number of verbal contexts (completive, incompletive, durative, and so on). Three of these forms are differentiated by the length and tonal patterns on their vowels. For class I, stems have short vowels and no special tonal accent. Class III stems have long vowels and falling tonal accents, and class IV have long vowels and no special accent. Morphological processes that affect the quality, quantity, or tonal patterns of vowels are often referred to as ablaut.

In German, when certain suffixes like the diminutive suffix -lein are added to a stem, the stem vowel becomes a front vowel. Historically, this fronting was a phonological process that occurred when a following suffix itself contained a front vowel; this process is called umlaut. Over time, the front vowels were lost in some suffixes or became back vowels, as is the case with the diminutive suffix -lein (pronounced [lain]).