In every language, the rules of morphology determine how phonemes can be combined to create morphemes.
Morphology is a study of language which proposes rules as to how sounds (phonemes) are brought together to create the smallest parts of words (morphemes) which are then combined to create words.
For example, anti- in antithesis is a morpheme - it is not a complete word (although sometimes morphemes can be) but it is rather its part created by the combination of written sounds, or phonemes.