text: Carbon is one of the few elements that can form long chains of its own atoms, a property called catenation. This coupled with the strength of the carbon–carbon bond gives rise to an enormous number of molecular forms, many of which are important structural elements of life, so carbon compounds have their own field of study: organic chemistry.
  Branching is also common in C−C skeletons. Carbon atoms in a molecule are categorized by the number of carbon neighbors they have:
  * A primary carbon has one carbon neighbor.
  * A secondary carbon has two carbon neighbors.
  * A tertiary carbon has three carbon neighbors.
  * A quaternary carbon has four carbon neighbors.
  In "structurally complex organic molecules", it is the three-dimensional orientation of the carbon–carbon bonds at quaternary loci which dictates the shape of the molecule.  Further, quaternary loci are found in many biologically active small molecules, such as cortisone and morphine.