Tone Unit

Tone Unit:
a small no. of particularly prominent syllables, in its smallest form may consist of only 1 syllable.

Tonic syllable:
a syllable which carries a tone.

Tonic Stress:
stress carried by the tonic syllable. some writers use terms (nucleus) and (nuclear stress) for the syllable / stress.

Speech----> no. of utterances ----> 1/more tone unit ----> 1/more feet ---(each)--> 1/more syllable ---(each)--->1/more phonemes.

Structure of Tone Unit: (PH - H - TS - T)

1. Head: all of that part of a tone-unit that extends from 1st stressed syllable up to tonic syllable. (not including TS). if there is no stressed syllable before the tonic syllable, there cannot be a head.
2. Pre-head: composed of all the unstressed syllables in a tone-unit preceding the 1st stressed syllable. found in 2 main environments: a)there is no head {ex: in an hour}. b)there is a head {ex: in a little less than an hour}.
3. Tail: syllables follow the tonic syllable. any syllables between tonic syllable and the end of the tone-unit.

Note:

• Tone-units are some times separated by silent pauses and sometimes not, pause type boundaries marked by(ll), non pause boundaries by (l).
• Tone is carried by the tonic-syllable.
• Intonation is carried by tone-unit.
• Tonic syllable is where the pitch movement of tone begins, but that pitch movement is  completed over the rest of the tone-unit (tail).

Source: Peter Roach book for phonetics and phonology