An ELT Glossary : Elements of the Tone Group

A tone group (or tone unit)  is a group of words centred around a tonic syllable (also called the nucleus) which carries a change in pitch. The five tones, ie pitch changes, in English are fall, rise, fall-rise, rise-fall and level. You can usually identify the change from one tone group to another by a slight pause by the speaker. Compare for instance the responses that might be made to the question Do you like sushi? :
//I don't \know//-  one tone group  - tonic syllable (with falling tone) = know
//I \don't//\no// - two tone groups - Tonic syllables (both with falling tone) don't and no.

(NB: Notice that the beginning and end of the tone group are indicated by double slashed brackets - // )

The tone group therefore has one obligatory element - the tonic syllable. However, it may also contain other optional elements - the pre-head, head and tail 

The term head refers to the section of a tone group that extends from the first stressed syllable (the onset) up to the tonic syllable or nucleus  Any unstressed syllables before the head are known as the prehead, while the syllables after the tonic are the tail.

So in the tone group  //He ˈwent to the Uniˈversity of  \Lancaster //  we have...

Pre-head : He
Head : went to the University of 
Tonic : Lan (here with a falling tone)
Tail : caster

As we said before, pre-head, head and tail are all optional elements. Potentially a tone group may consist of the nucleus (tonic syllable) only : 
// \Look! //

In all of the following tone groups, one or more of the optional elements are missing :

//Don''t \look //  =  Prehead + tonic syllable

//\Look over ˈthere//  = Tonic syllable + tail

and with contrastive stress (ie in reply to David's over there)

//ˈDavid \isn't over ˈthere // = Head + tonic syllable + tail