"" is the phenomenon that a verb which in PIE had a stem ending in one of these phonemes displays a differing reflex in different parts of the paradigm, a result of the movable nature of accent in PIE. The Germanic past tense derives from the PIE perfect aspect, which was always athematic and therefore almost always had a shift of accent between the singular indicative (where it was on the root syllable) and the remaining forms including the past participle (where it was on the ending). However, the perfect aspect was only present in primary, underived verbs, and any derived verbs therefore lacked perfect forms altogether. These latter verbs formed the base of the Germanic weak verbs, and did not inherit the accent shift, so the alternation itself only affects Germanic strong verbs.