By default, when ribbon is shown, mainchain atoms (backbone atoms) are hidden and their locations are adjusted to fall on the ribbon.
Ribbackbone allows all backbone atoms to be displayed at their unadjusted positions together with ribbon (of course, these atoms can still be undisplayed). ~Ribbackbone restores the default behavior. 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 residues within a model can also be controlled in the molecule model attributes panel.
By default, when ribbon is shown:
Of the amino acid mainchain atoms N, CA, C, and O, only CA is displayed within most residues (because it is bonded to side chain atoms). Some N and C atoms remain displayed: 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).
Of the nucleic acid mainchain atoms P, O1P, O2P, O5', C5', and O3', only C5' and O3' are displayed within most residues (because they are bonded to C4' and C3', respectively). In addition, the P atom of the first residue in each stretch of ribbon (unless it is the first residue in a chain) remains displayed.The ribbon residue class determines which atoms are considered mainchain. The amino acid and nucleic acid classes are built into Chimera, but additional classes can be created and named with the Residue Class section of the Ribbon Style Editor.
See also: ribbon, ribrepr, ribscale, display