The ribbackbone command allows ribbon and all backbone atoms to be displayed simultaneously for residues within a model. The standard names for backbone atoms are N, CA, C, and O. The ~ribbackbone command disallows simultaneous display: when ribbon is shown for residues in the model, all O atoms and most N and C atoms in those residues will disappear; only the CA atoms, the N atom of the first residue in each stretch of ribbon (unless it is the first residue in a chain), and the C atom of the last residue in each stretch of ribbon (unless it is the last residue in a chain) can be displayed. Of course, any of these atoms can be undisplayed (with ~display).
If the atom-spec is blank, all open molecule models will be affected; otherwise, the entire model(s) containing atom-spec will be affected. Whether ribbon and backbone atoms can be displayed simultaneously for the same residue can also be controlled in the molecule model attributes panel.
See also: ribbon