Error: invalid conversion from 'char **' to 'const char **'

Question

I searched around, and so far I have not found a satisfactory answer to this question.

Why are these compilations:

char c; char const * d = &c; char e; char * const f = &e; char* g; char* const * h = &g; 

But it is not?

 char* a; char const ** b = &a; 

error: invalid conversion from 'char **' to 'const char **'