Two or more segments are tautosyllabic (with each other) if they occur in the same syllable. For instance, the English word "cat" () is monosyllabic and so its three phonemes , and are tautosyllabic. They can also be described as sharing a 'tautosyllabic distribution'.
Phonemes that are not tautosyllabic are heterosyllabic. For example, in the English word "mustard" , and are heterosyllabic since they are members of different syllables.