  We investigate various aspects of operator quantum error-correcting codes or,
as we prefer to call them, subsystem codes. We give various methods to derive
subsystem codes from classical codes. We give a proof for the existence of
subsystem codes using a counting argument similar to the quantum
Gilbert-Varshamov bound. We derive linear programming bounds and other upper
bounds. We answer the question whether or not there exist
[[n,n-2d+2,r>0,d]]<sub>q</sub> subsystem codes. Finally, we compare stabilizer
and subsystem codes with respect to the required number of syndrome qudits.
