Residua macromolecules can be approximated by various combinations of two building blocks that are linked by thermally labile bonds. The pendant building block is volatile when cracked off the rest of the residua molecule while the core building block remains nonvolatile. This simple model explains why the Conradson carbon residue, the standard measure of coking tendency, is a linear function of hydrogen content for petroleum molecules with molecular weights greater than 700 amu and why the cumulative properties of the distillable liquid products of the thermal conversion of residua are independent of conversion. In addition, the model provides a basis for calculating the change in the total Conradson carbon residue with processing of residua, including that it is unaltered by separation, decreased by hydroconversion and increased by thermal conversion. The hydrogen contents of the pendant building block, 11.6 +/- 0.4 wt %, and of the core building block, 3.8 +/- 0.3 wt %, were determined by two different experimental techniques each and found to be consistent, using six vacuum residua and three heptane-soluble fractions. Finally, pendant-core model compounds were found to have Conradson carbon residues similar to vacuum residua and their heptane-soluble fractions. These suggest that even one-ring aromatic cores can produce Conradson carbon residue if multiple bonds need to be broken before a core can volatilize.