User blog comment:PsiCubed2/Letter Notation Part II/@comment-30675002-20161225090641/@comment-30754445-20161225094329

There are no A,B,C or D in the original notation, because it is an extension of ordinary scientific notation (which uses E to denote powers of 10).

But it is possible to extend the notation downward:

Dx = 10x

Cx = x+10

Bx = x+2

Ax = x/2-1

This would preserve the rule of 2= 10, and also give us A2=0 as the starting point (another option is to have Ax=x, which is more intuitive at the cost of breaking the above rule)

As for what comes after Zebra: 'Z' is supposed to represent a very large ordinal in the FGH (something on the order of collapsed Mahlo cardinals), so that question won't be relevant for quite some time. It should also be noted that once we go beyond P or Q, the system becomes far less intuitive, which kinda betrays the whole point of inventing it.