Source : Webster's Revised Unabridged Dictionary (1913)

Ad- \Ad-\ [A Latin preposition, signifying to. See {At}.]
   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.