With fonological opacity, the phenomenon is meant that the operation of a phonological line in the surface form of one or more words is not visible because another phonological process occurs in the same context.
Fonological opacity is closely linked to regulation order (see feed order and bleeding order for more explanation). The phenomenon plays an important role in the Optimal Theory. Examples
An example can be seen in the dialect of Aalst and some neighboring dialects. Here, in certain contexts, regressive assimilation of nasals (as sandhi) and deletion of the sjwa takes place simultaneously. Because the first rule has rolled earlier than the latter, the effect of regressive nasal assimilation is not visible in [ɣɾy: n bʋəmən], "green trees" (underlying: [ɣɾy: nə bʋəmən]) no longer visible. Had progressive assimilation of nasal end sounds and sjwa deletion been reversed here, then there had been a nutritional order of rules and the effect of the first line was visible. This is actually the case for, for example, the form [ɣɾy: m bʋəməkə], "green tree", where no sjwa deletion has occurred.
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