In PIE, causative verbs (meaning "to cause to") were derived from verb roots with a suffix *"-éye-", and the root vowel was changed to the "o"-grade. Verbs with this suffix eventually became part of the first weak class (*"-jan" verbs). This suffix always bore the accent, and the verb root never did, while in regular strong verbs the verb root was accented in the present tense. This caused Verner alternation between the original verbs and the causative verbs derived from them.