Joseph Wright, A Middle High German Primer
“ In MHG. there are six such series, which appear most clearly in the stem-forms of strong verbs. Four stem-forms are to be distinguished in a MHG. strong verb which has vowel gradation as the characteristic mark of its different stems:— (1) the present stem, to which belong all the forms of the present, (2) the stem of the first or third person of the preterite singular, (3) the stem of the preterite plural, to which belong the second person of the preterite singular and the whole of the preterite subjunctive, (4) the stem of the past participle. ”
