Allophonic assimilation overview
Consider the following phrase.
There is a word boundary between the word bad and the word thought. The sound at the end of the word bad is the alveolar /d/ and the sound at the beginning of the word thought is the dental fricative /θ/. Now, just as alveolars may become dentalized within a word, the same process of assimilation may occur across a word boundary. Consequently, the phrase may be realized as follows.
In this instance, the final /d/ of bad is dentalized in anticipation of the upcoming dental /θ/ that initiates the immediately following word thought. The alveolar /d/ is said to assimilate the place of articulation of the neighboring dental fricative. This is an example of allophonic assimilation because the new, transformed sound is an allophone of the original phoneme, i.e. [d̪] is an allophone of /d/.
We will consider four types of allophonic assimilation in connected speech: