past indicative singular (part 2) in the o-grade, and remaining past (part 3) and past participle (part 4) in the zero grade. The differences between classes 1, 2, and 3 arise from semivowels coming after the root vowel, as shown in the table below. As can be seen, the e-grade in part 1 and o-grade in part 2 are shared by all of these five classes. The difference between them is in parts 3 and 4: In classes 1 and 2, the semivowel following the vowel was converted in the zero grade into a full vowel. In class 3 and the past participle of class 4, there was no semivowel but there were PIE syllabic resonants which developed in Germanic to u plus resonant; thus u became the Germanic sign of these parts. There is some evidence that this may have been the original behaviour of the past nonsingular / nonindicative of class 4 as well: to wit, preterite-presents whose roots have the class 4 shape show u outside the present indicative singular, such as *man- ~ *mun- "to remember", *skal- ~ *skul- "to owe". In class 5, the zero grade of the past participle had probably been changed to e-grade already in PIE, because these verbs had combinations of consonants that were phonotactically illicit as a word-initial cluster, as they would be in the zero grade. The *ē in part 3 of classes 4 and 5 is not in fact a PIE lengthened grade but arose in Germanic. Ringe suggests that it was analogically generalised from the inherited part 3 of the verb *etaną "to eat" before it had lost its reduplicant syllable, PIE *h1eh1d- regularly becoming Germanic *ēt-. Class 6 appears in Germanic with the vowels a and ō. PIE sources of the a vowel included *h2e, *o, and a laryngeal between consonants; possibly in some cases the a may be an example of the a-grade of ablaut, though the existence of such a grade is controversial. It is not clear exactly how the ō is to be derived from an earlier ablaut alternant in PIE, but believable sources include contraction of the reduplicant syllable in PIE *h2-initial verbs, or o-grades of verbs with interconsonantal laryngeal. In any event, within Germanic the resulting a ~ ō behaved as just another type of vowel alternation. In Proto-Germanic, this resulted in the following vowel patterns: Class 2b is of unknown