Ribbon shows a ribbon representation of the secondary structure of proteins and nucleic acids; ~ribbon turns off the ribbon display. Even if only particular atoms are specified, the ribbon segment(s) corresponding to the entire residue(s) containing the atoms will be affected. The style of ribbon can be controlled with ribrepr. Ribbon display and style can also be controlled with the Actions menu and the molecule model attributes panel.
By default, certain backbone atoms are hidden when ribbon is displayed; the command ribbackbone can be used to allow simultaneous display of ribbon and backbone atoms.
Protein secondary structure is determined from HELIX and SHEET records in the input PDB file (see the Intro to PDB Format (PDF)) or by using ksdssp. For nucleic acids, the ribbon simply follows the phosphodiester backbone. No ribbon is drawn for molecules that are not proteins/peptides or nucleic acids.
The color of the ribbon is assigned on a per-residue basis (for example, using ribcolor or rainbow) and may differ from the apparent color of the residue's atoms; see coloring hierarchy. If ribbon colors have not been assigned, the ribbon defaults to the model-level color.
See also: ribbackbone, ribcolor, ribrepr, ribbonjr, the Ribbon Style Editor