to what refers E'_G?

File in which the problem appears

12 band structures

Problematic sentence

The band structure has two band gaps: direct, the band gap at k=0, E'_G ...

Correct version

I suspect E'_G has to do with the direct band gap, but it quite hard to derive as it is written. The first few times I read it, 'E'_G and indirect gap E_G' became the main thing I read, which does not seem to be intended.