Meaning of 'ad-' (Webster Dictionary)
- 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.