Dissimilation is the phonological process by which two similar sounds in a word become less alike, typically because producing two near-identical segments in close succession is articulatorily and perceptually awkward. It is the mirror image of assimilation and is usually attributed to the horror aequi principle: speakers avoid repetition of identical or near-identical structures.
The most common type in English involves the liquids /r/ and /l/. When a word contains two of these sounds, one often shifts to break the similarity. The classic case is pilgrim: Latin peregrinus "foreigner" passed through Late Latin pelegrinus, where the first /r/ dissimilated to /l/ under the influence of the second /r/, and entered English via Old French. Colonel shows the reverse pattern. The word was borrowed twice: once as coronel from French (which had itself dissimilated Italian colonnello), and once as the spelled form colonel. Modern English compromised — keeping the spelling of one variant but the pronunciation of the other — yielding /ˈkɜːnəl/ in RP and /ˈkɝnəl/ in General American. Marble (Old French marbre < Latin marmor) and purple (Old English purpure < Latin purpura) show the same /r-r/ → /r-l/ pattern.
Dissimilation also affects other features. Voicing dissimilation appears in Greek trikhos (genitive of thrix "hair"), where original aspiration on both fricatives could not coexist (Grassmann's Law). Nasal dissimilation shows up in some dialectal pronunciations of chimney as chimley. Vowel dissimilation is rarer but attested historically in the development of Romance vowel systems.
Dissimilation can be local (between adjacent segments) or long-distance (across syllables), and either anticipatory (the later sound triggers change in the earlier one) or perseverative. Crucially, while assimilation applies almost everywhere in connected speech and is highly regular, dissimilation is sporadic and lexically specific. It tends to leave residues — single words affected — rather than systematic patterns.
Dissimilation rarely surfaces as a teaching point because it is not a productive process in modern English. It explains, however, why certain spelling-pronunciation mismatches exist (colonel, pilgrim, marble) and why a few Latinate or Romance borrowings have unexpected shapes. For learners puzzled by colonel /ˈkɜːnəl/, a brief etymological note removes the mystery and reduces the pull of spelling pronunciation. Beyond such individual lexical items, dissimilation has little classroom application; it belongs to the historical background that shapes the current inventory rather than to active pronunciation work.