changed or disappeared, because the umlaut allophones gradually shifted to such a degree that they became distinctive in the phonological system of the language and contrastive at a lexical level. However, sporadic place-name attestations demonstrate the presence of the secondary umlaut already for the early 9th century, which makes it likely that all types of umlaut were indeed already present in Old High German, even if they were not indicated in the spelling. Presumably, they arose already in the early 8th century. Ottar Grønvik, also in view of spellings of the type , , and in the early attestations, affirms the old epenthesis theory, which views the origin of the umlaut vowels in the insertion of after back vowels, not only in West, but also in North Germanic. Fausto Cercignani prefers the assimilation theory and presents a history of the OHG umlauted vowels up to the present day. In modern German, umlaut as a marker of the plural of nouns is a regular feature of the language, and although umlaut itself is no longer a productive force in German, new plurals of this type can be created by analogy. Likewise, umlaut marks the comparative of many adjectives and other kinds of derived forms. Because of the grammatical importance of such pairs, the German umlaut diacritic was developed, making the phenomenon very visible. The result in German is that the vowels written as , , and become , , and , and the diphthong becomes : "man" vs. "men", "foot" vs. "feet", "mouse" vs. "mice". In various dialects, the umlaut became even more important as a morphological marker of the plural after the apocope of final schwa (); that rounded front vowels have become unrounded in many dialects does not prevent them from serving as markers of the plural given that they remain distinct from their non-umlauted counterparts (just like in English foot – feet, mouse – mice). The example "guest" vs. "guests" served as the model for analogical pairs like "day" vs. "days" (vs. standard ) and "arm" vs. "arms" (vs. standard ). Even plural forms like "fish" which had never had a front rounded vowel in the first place were interpreted as such (i.e., as if from Middle High German **) and led to singular forms like that are attested in some dialects. I-mutation in Old Saxon In Old Saxon, umlaut is much less apparent than in Old Norse.