Qualitative rules governing carbon and bridge-hydrogen placement permit the prediction of the most stable isomeric structures for the various carboranes. Seventy-three isomeric boron hydride and carborane structures, from B6H9- to C4B2H6, were computed at the ab initio MP2(fc)/6-31G* level to determine their relative stabilities quantitatively. Specific architectural features, recognized to be unfavorable, were assigned "energy penalty" values that allow the projection of comprehensive thermodynamic stability values via a simple additivity procedure. These values match the ab initio results with surprising precision. Our study includes Siebert's nido-2,3,5-C3B3H7 and Wrackmeyer's nido-2,4-C2B4H8 nido-6 carboranes, which contain "unusual" CH-B-bridge hydrogens.