all that / that

[Mnemon]

a. He's not that clever.

b. He's not all that clever.

What's the difference?

 

[jutfrank]

The word that in sentence a. could be emphatic—referring comparatively to what has just been said. (He's not as clever as that.) Sentence b. can't do that.

Equally, both that and all that could also be degree modifiers, similar to 'very'. (He's not very clever.)