as a prefix ad- assumes the forms ac-, af-, ag-, al-, an-, ap-, ar-, as-, at-, assimilating the d with the first letter of the word to which ad- is prefixed. it remains unchanged before vowels, and before d, h, j, m, v. examples: adduce, adhere, adjacent, admit, advent, accord, affect, aggregate, allude, annex, appear, etc. it becomes ac- before qu, as in acquiesce.