ad-

(Latin: to, a direction toward, addition to, near; at; used as a prefix)

The Latin element ad carries the idea of "in the direction of" and combines with many Latin words and roots to make common English words.

Notice that the d of ad usually changes into the same letter as the first letter of the following root or word when it is a consonant: ad-fix becomes affix, and ad-sign becomes assign; therefore, making a double consonant.

Another example includes: attract is ad-tract (drawn towards); so it has a double t. On the other hand adapt is simply ad-apt, with one d. For the same reason, there is only one d in adore and adumbrate, because ad- has combined with orare and umbra.

Since these Latin words begin with vowels and not consonants, the d of ad does not double; even with other letter formats as shown previously.