That has led to a controversy over when and how i-mutation appeared on these vowels. Some (for example, Herbert Penzl) have suggested that the vowels must have been modified without being indicated for lack of proper symbols and/or because the difference was still partly allophonic. Others (such as Joseph Voyles) have suggested that the i-mutation of and was entirely analogical and pointed to the lack of i-mutation of these vowels in certain places where it would be expected, in contrast to the consistent mutation of . Perhaps the answer is somewhere in between — i-mutation of and was indeed phonetic, occurring late in OHG, but later spread analogically to the environments where the conditioning had already disappeared by OHG (this is where failure of i-mutation is most likely). It must also be kept in mind that it is an issue of relative chronology: already early in the history of attested OHG, some umlauting factors are known to have disappeared (such as word-internal "j" after geminates and clusters), and depending on the age of OHG umlaut, that could explain some cases where expected umlaut is missing.