In linguistics, alternation is a variation in the form and/or sound of a word or word part. (Alternation is equivalent to allomorphs in morphology.) Also known as alternance. A form involved in an alternation is called an alternant. The customary symbol for alternation is ~. American linguist Leonard Bloomfield defined an automatic alternation as one that's "determined by the phonemes of the accompanying forms" ("A Set of Postulates for the Science of Language," 1926). An alternation that affects only some morphemes of a particular phonological form is called non-automatic or non-recurrent alternation.

2. • In linguistics, alternation is a variation in the form and/or sound
of a word or word part. (Alternation is equivalent to allomorphs
in morphology.) Also known as alternance.
• A form involved in an alternation is called an alternant.The
customary symbol for alternation is ~.
• American linguist Leonard Bloomfield defined an automatic
alternation as one that's "determined by the phonemes of the
accompanying forms" ("A Set of Postulates for the Science of
Language," 1926). An alternation that affects only
some morphemes of a particular phonological form is
called non-automatic or non-recurrent alternation.

4. • "Certain English nouns ending in the consonant /f/ form their plurals with
/v/ instead: leaf but leaves, knife but knives.We say that such items exhibit
an /f/-/v/ alternation. . .
• "A somewhat different alternation is found in related words like electric
(which ends in /k/) and electricity (which has /s/ instead of /k/ in the same
position).
• "More subtle is the three-way alternation occurring in the English plural
marker.The noun cat has plural cats, pronounced with /s/, but dog has
plural dogs, pronounced with /z/ (though again the spelling fails to show
this), and fox has plural foxes, with /z/ preceded by an extra vowel.This
alternation is regular and predictable; the choice among the three
alternants (as they are called) is determined by the nature of the
preceding sound."

5. • "[T]ypically, an allomorphic alternation makes the most sense phonologically if
one looks at an earlier stage of the language. Here are [five] striking examples:
• Foot feet
goose geese
tooth teeth
man men
mouse mice
• In this list of words, the different vowels in the plural arose in Prehistoric English.
At that time, the plurals had an /i/ ending. English also had a phonological rule
(known by the German word umlaut) whereby vowels preceding an /i/ became
closer to the /i/ in pronunciation.At a later date, the ending was lost. In terms of
the phonology of Modern English, the current allomorphy is doubly senseless.
First, there is no overt ending to explain the alternation in the stem. Second, even
if there were, English has lost the umlaut rule. For example, we feel no pressure at
all to turn Ann into xEnny when we add the suffix -y/i/.

7. • "The grammatical category of voice affords speakers some flexibility in
viewing thematic roles. Many languages allow an opposition between
active voice and passive voice.We can compare for example the English
sentences in 6.90 below:
In the active sentence 6.90a Billy, the agent, is the subject and the
horses, the patient, is the object. The passive version 6.90b,
however, has the patient as the subject and the agent occurring in
a prepositional phrase ... This is a typical active-passive
voice alternation: the passive sentence has a verb in a different
form — the past participle with the auxiliary verb be--and it allows
the speaker a different perspective on the situation described."

8. • "According to Langacker (1987: 218), predicative adjectives have a
relational profile: they convey a quality, which functions as the landmark
(lm) in the reduction, that is associated with the entity denoted by the
subject of the utterance, which is the trajectory (tr). Consequently, only
elements with a relational profile can be used as predicates. Applied to
the discussion of grounding elements, this entails that alternation with a
predicative construction is only available for elements that
express deictic meanings but profile the grounding relation, e.g. a
known criminal - a criminal that is known, and not for grounding
predications, which have a nominal profile. As shown in (5.28),
comparative determiner units do not allow alternation with the
predicative construction, which suggests them to have a nominal rather
than a relational profile: