We can turn 9-9-9 into a progressive tax system very simply.

1) 9% sales tax: exclude groceries, meds, rents, utilities.
That's where the poor spend most of their dollars.
(A reminder: 9-9-9 eliminates regressive payroll taxes, thus removing a huge impediment to hiring.)
Taxing consumption instead of labor is a fundamental improvement to our tax system.

2) Exempt the first $20k from the 9% income tax.
This is especially helpful to minimum wage workers, temps and youth.

3) Exempt the first $50k from the 9% corporate tax.
Now you're helping small biz.
That's where most new jobs come from.

The simplicity of 9-9-9 affords transparency.
Simplicity makes it tough to sneak in pet exemptions or increases.
Unlike under our current tax code, going from 9-9-9 to 10-10-10 would be hard without everybody noticing.