Allomorphs and Their Distributions

suppletion. Besides phonological allomorphs, morphemes may also have allomorphs that are not at all similar in pronunciation. These are called suppletive allomorphs. For instance, the English verb go has the suppletive stem wen in the past tense (wen-t), and the English adjective good has the suppletive stem bett in the comparative degree (better).
It is not always easy to decide whether an alternation is phonological or suppletive, because the categories are end points on a continuum of traits, rather than a clear-cut binary distinction. Some examples are therefore intermediary. For instance, what about English buy/bought, catch/caught, teach/taught? The root allomorphs of these verbs ([bai]/[ bɑːt ], [kæt∫]/[ kɑːt], [ tiːtʃ ]/[ tɑːt ]) are not as radically different as go/wen-t, but they are not similar enough to be described by phonological rules either. In such cases, linguists often speak of weak suppletion, as opposed to strong suppletion in cases like go/went, good/better.
Just for addition in brief allomorphy can be devided into 3 :

1. 
   Phonological allomorphy which the alternation could be described by a rule of pronunciation
   

For example : English plural [-z], [-s],[-əz];

2. 
   Weak suppletive allomorphy is allomorphs that exhibit some similarity, but this cannot be described by phonological rules
   

For example : English buy/bough-, catch/caugh-, etc.

3. 
   Strong suppletive allomorphy is allomorphs that exhibit no similarity at all
   

For example : good/bett