Spindle: Most antique doorknobs are found in pairs, screwed onto either end of a long rod or square spindle. When the original spindle is missing, replacements can be found without much difficulty through architectural salvage sources or hardware stores.
Base: Knob bases are often set into rosettes, or circular plates, that can be unadorned or ornamental.
Knob: In addition to age and degree of detail, the material a doorknob is made from can dramatically affect desirability. Clear glass, porcelain and decorative brass are fairly common.