are more words of B than before. there is at least one environment for which A, for the time being, no longer occurs, called a gap in the distribution of the phoneme. there is, under certain circumstances, an alternation between A and B if inflection or derivation result in A sometimes but not always being in the environment in which it merged with B. Example from Middle English For a simple example, without alternation, early Middle English /d/ after stressed syllables followed by /r/ became /ð/: módor, fæder > mother, father /ðr/, weder > weather, and so on. Since /ð/ was already a structure-point in the language, the innovation resulted merely in more /ð/ and less /d/ and a gap in the distribution of /d/ (though not a very conspicuous one). Note 1: thanks to borrowing, from dialects as well as other languages, the original distribution has been disturbed: rudder, adder in Standard English (but forms with /ð/ are attested in nonstandard dialects). Note 2: one who knows German can figure out which cases of English /ð/ were originally /ð/ and which changed from /d/. Original /d/ corresponds to /t/ in German, and original /ð/ corresponds to /d/. Thus, leather = German Leder, brother = Bruder, whether = weder, wether = Widder, pointing to original /ð/ in English; weather = German Wetter, father = Vater, mother = Mutter pointing to original /d/. Note 3: alternation between /d/ and /ð/ would have been a theoretical possibility in English, as in sets like hard, harder; ride, rider, but any such details have been erased by the commonplace diachronic process called morphological leveling. Devoicing of voiced stops in German A trivial (if all-pervasive) example of conditioned merger is the devoicing of voiced stops in German when in word-final position or immediately before a compound boundary (see: Help:IPA/Standard German): *hand "hand" > /hant/ (cf. plural Hände /ˈhɛndə/) Handgelenk "wrist" /ˈhantgəlɛŋk/ *bund "league, association" > /bʊnt/ (cf. plural Bünde /ˈbʏndə/) *gold "gold" > /gɔlt/ *halb "half" > /halp/ (cf. halbieren "to halve" /halˈbiːʁən/) halbamtlich "semi-official" /ˈhalpʔamtlɪç/ *berg "mountain" /bɛɐ̯k/ (cf. plural Berge /ˈbɛɐ̯gə/) *klug "clever, wise" > /kluːk/ (cf. fem. kluge /ˈkluːgə/) There were, of course, also many cases of original voiceless stops in final position: Bett "bed", bunt "colorful", Stock "(walking) stick, cane". To sum up: there are the same number of structure points as before, /p t k b d g/, but there