Answer:

Assimilation in linguistics refers to the phonological process where a sound in a word becomes more similar to a neighboring sound, influenced by articulatory or physiological factors. This phenomenon occurs in connected speech, compounds, and even words and plays a significant role in natural pronunciation. It reflects the ease of articulation and can affect both consonants and vowels, leading to changes in pronunciation.  

For example, in isolation the words ‘is’ and ‘she’ are pronounced [ɪz] and [ʃi] respectively; but in connected speech they are often pronounced [ɪʒʃi] rather than [ɪzʃi].

Rules of assimilation: There are various types of assimilation, and each type has its own set of rules or patterns. They are described below.

1. Progressive (or left-to-right) Assimilation: Progressive assimilation is when one sound in a sequence affects the pronunciation of the sound that follows it, often moving from left to right.
   For example:
   • “dogs” : /s/ → [z] / — The ending [s] is assimilated to [z] by the influence of the preceding voice [g].
   • “pubs” : /s/ → [z] / — The ending [s] is assimilated to [z] by the influence of the preceding voice [b].

2. Regressive, or Anticipatory (or right-to-left) Assimilation: Regressive assimilation, also known as anticipatory assimilation or right-to-left assimilation, involves a sound being influenced by a following sound. This can lead to phoneme changes, such as in “miss you” where [s] becomes [S] due to the influence of [ j ].

3. Reciprocal Assimilation: Reciprocal assimilation in linguistics occurs when two phonemes mutually influence each other, causing changes in both sounds.
   • For instance:
     • “Soldier” -> /’soʊldʒər/ 
     • “Picture” -> /’pɪkʧər/ 
     • “Fissure” -> /’fɪʃər/

4. Consonant Cluster Assimilation: Consonant assimilation is a phonological process where nearby sounds change and can even fuse, forming geminate consonants. Examples include:
   • Before velar consonants (/k/ and /g/):
     • /n/ → /ŋ/ (e.g., “ten cars”)
     • /d/ → /g/ (e.g., “good girl”)
     • /t/ → /k/ (e.g., “that kid”)
   • Before bilabial sounds (/m/, /b/, /p/):
     • /n/ → /m/ (e.g., “ten men” /ten men/)
     • /d/ → /b/ (e.g., “bad boys”)
     • /v/ → /p/ (e.g., “hot mushrooms,” “that person,” “meat pie,” “light blue”)
       
   1. Place Assimilation: Consonants alter their articulation to match neighboring sounds. For example, in “impossible,” the /m/ sound changes its place of articulation to match the /p/ sound, resulting [ɪmˈpɑsəbəl].
   2. Voice Assimilation: Consonants change their voicing based on the voicing of neighboring sounds. For example, the plural morpheme “-s” is pronounced as /z/ when the preceding sound is voiced, as in the word “dogs” [dɔɡz].
   3. Manner Assimilation: Consonants change their manner of articulation to be more similar to a neighboring sound. For example, in the phrase “ten schools,” the /n/ sound changes to [m] due to the influence of the following bilabial sound /s/. 

5. Vowel Assimilation: Vowels can also be subject to assimilation, where they become more similar to neighboring vowels.
   • Height Assimilation: Vowels adjust their height (openness) to match nearby vowels. For instance, in fast speech, “good boy” can be pronounced as [ɡʊd bɔɪ], with both vowels becoming more central.
   • Backness Assimilation: Vowels modify their backness to match nearby vowels. For example, in the word “impossible,” the final /ə/ might be pronounced as [ɪmˈpɑsɪbl] due to the influence of the /ɪ/ sound.

6. Total Assimilation: In some cases, total assimilation occurs when a sound completely changes to match a neighboring sound. For example, in fast speech, “handbag” might be pronounced as [hæmbæɡ], where the /n/ completely assimilates into the /m/ sound.

In conclusion, assimilation in language isn’t always mandatory and can depend on factors like speech rate, formality, and emphasis. Linguists study assimilation to grasp sound change patterns and their role in shaping a language’s phonological system.