Ad-

CIDE DICTIONARY

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.  [1913 Webster]

OXFORD DICTIONARY

Ad-, prefix (also a- before sc, sp, st, ac- before c, k, q, af- before f, ag- before g, al- before l, an- before n, ap- before p, ar- before r, as- before s, at- before t)
1 with the sense of motion or direction to, reduction or change into, addition, adherence, increase, or intensification.
2 formed by assimilation of other prefixes (accurse; admiral; advance; affray).

Etymology

(sense 1) (through OF a-) f. L ad to: (sense 2) a- repr. various prefixes other than ad-