verbs, this has the same vowel as part 1. When it is distinct, as here, it is always derived from part 1 by Umlaut. For this reason, some textbooks do not treat it as a principal part. The preterite (i.e. past indicative) 1st singular: , which is identical to the 3rd singular. In this verb, part 3 comes from a PIE o-grade. The preterite plural: . In West Germanic, the same vowel is used in the 2nd singular. In this verb, part 4 comes from a PIE zero-grade. The past participle: . This vowel is used only in the participle. In some verbs, part 5 is a discrete ablaut grade, but in this class 2 verb it is derived from part 4 by an a-mutation. Strong verb classes Germanic strong verbs are commonly divided into 7 classes, based on the type of vowel alternation. This is in turn based mostly on the type of consonants that follow the vowel. The Anglo-Saxon scholar Henry Sweet gave names to the seven classes: The "drive" conjugation The "choose" conjugation The "bind" conjugation The "bear" conjugation The "give" conjugation The "shake" conjugation The "fall" conjugation However, they are normally referred to by numbers alone. In Proto-Germanic, the common ancestor of the Germanic languages, the strong verbs were still mostly regular. The classes continued largely intact in Old English and the other older historical Germanic languages: Gothic, Old High German and Old Norse. However, idiosyncrasies of the phonological changes led to a growing number of subgroups. Also, once the ablaut system ceased to be productive, there was a decline in the speakers' awareness of the regularity of the system. That led to anomalous forms and the six big classes lost their cohesion. This process has advanced furthest in English, but in some other modern Germanic languages (such as German), the seven classes are still fairly well preserved and recognisable. The reverse process in which anomalies are eliminated and subgroups reunited by the force of analogy is called "levelling", and it can be seen at various points in the history of the verb classes. In the later Middle Ages, German, Dutch and English eliminated a great part of the old distinction between the vowels of the singular and plural preterite forms. The new uniform preterite could be based on the vowel of the old preterite singular, on the old plural, or sometimes on the participle. In