The bond orbitals, associated with the Lewis structure, can be used as the basis set of canonical molecular orbitals. The concept of linear combination of bond orbitals for canonical molecular orbitals provides a link between two seemingly rival theories: valence bond theory and molecular orbital theory.The bond orbital basis set can reveal the character of the canonical molecular orbitals and the orbital delocalization due to intramolecular interaction. An index, called Lewis bond order, defined as the weight of bonding type bond orbitals, can be used to account for the bonding nature of the canonical molecular orbitals and thus also account for the vibrational structures of photoelectron spectroscopy. Each canonical molecular orbital of the molecules HCN and C 2 H 2 may have several Lewis bond orders corresponding to several bonds in the Lewis structure.