Selective gas etching for self-aligned pattern transfer uses a first block and a separate second block formed in a sacrificial layer to transfer critical dimensions to a desired final layer using a selective gas etching process. The first block is a first hardmask material that can be plasma etched using a first gas, and the second block is a second hardmask material that can be plasma etched using a second gas separate from the first gas. The first hardmask material is not plasma etched using the second gas, and the second hardmask material is not plasma etched using the first gas.