I-mutation is visible in Old High German (OHG), c. 800 AD, only on short , which was mutated to (the so-called "primary umlaut"), although in certain phonological environments the mutation fails to occur. By then, it had already become partly phonologized, since some of the conditioning and sounds had been deleted or modified. The later history of German, however, shows that and , as well as long vowels and diphthongs, and the remaining instances of that had not been umlauted already, were also affected (the so-called "secondary umlaut"); starting in Middle High German, the remaining conditioning environments disappear and and appear as and in the appropriate environments.