Nuclear tone

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Hello...

What is “nuclear tone”?

It seems to be much used, I can find resources that explain it.

Below are examples in the book, A STUDENT'S GRAMMAR OF THE ENGLISH LANGUAGE by Greenbaum and Quirk. But this book also never explains what “nuclear tone” is.

There seems to be, head, and tail in the nuclear tone. But, what it exactly is is difficult to find out.

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The nuclear tone of the tag occurs on the operator and is either rising or falling.

The particle of a phrasal verb is normally stressed, and in final position normally bears the nuclear tone, whereas the particle of a prepositional verb is normally unstressed and has the 'tail' of the nuclear tone that falls on the lexical verb:
Which friends did she CALL on?
~ Which friends did she call UP?

A positive imperative can be made more persuasive or insistent (esp in BrE) by adding do (usually with a nuclear tone) before the verb: DO have some more tea. DO let's go for a walk.