Indo-European roots seem to come in forms with either the vowel e or the vowel o, referred to as the e-grade form and the o-grade form, respectively. Roots with neither an e nor an o were referred to as zero-grade forms. In the case of word the zero-grade form was represented as wr-, The r used in the AHD transcription WO'N copied got lost in the copy. The suffix, -dh, didn't.