The paper takes the German noun plural formation as an example for different cases of doubt and shows in which circumstances German speakers can have problems to create the "normal" plural form corresponding to the standard. The paper distinguishes between native and non native plural forms. The cases of doubt within the former can be shown to result from either natural change which leads to a reduction of plural classes and explains the decline of the er- and the umlaut plural, or from a strategy of compensation which replaces the non iconic 0-plural by forms in -n or -s. The problems with the non native nouns varying between a plural form in -s and one ending in a schwa suffix are shown to be the consequence of the ongoing assimilation process. The -s is in complementary distribution with the native schwa suffixes of German and a means of integration: The non-syllabic -s allows for highly corresponding, "conservative" forms similar to the singular by adding only a segment, not a syllable to the stem. By contrast, a native plural with a syllabic suffix alters more or less severely the phonological shape of the base. The alterations yield a continuum of similarity that are demonstrated in a diagram. The paper shows that this distribution is functionally motivated. Loan words, nouns not yet fully established in the speech community, can only be altered in their phonological structure after they have gained a certain degree of familiarity. Only the established borrowings apply -en or -e because these allow "better" plural forms as to prosodic and phonological constraints, i.e. trochaic forms without consonant clusters and superheavy syllables. Finally, the paper discusses the consequences of this variation for the layers of the lexicon and for the grammar of German. Three appendixes indicate the statistical development of the plural classes during the last century, token frequencies of some examples and regional differences in the use of the variants.