A significant subclass of Class I weak verbs are (deverbal) causative verbs. They are formed in a way that reflects a direct inheritance from the PIE causative class of verbs. PIE causatives were formed by adding an accented affix "-éy-" to the "o"-grade of a non-derived verb. In Proto-Germanic, causatives are formed by adding a suffix "-j/ij-" (the reflex of PIE "-éy-") to the past-tense ablaut (mostly with the reflex of PIE "o"-grade) of a strong verb (the reflex of PIE non-derived verbs), with Verner's Law voicing applied (the reflex of the PIE accent on the "-éy-" suffix):