Valence bond theory is an empirically derived theory that describes how orbitals overlap in molecules to form bonds. When the bond forms, the probabiity of finding electrons changes to become higher within the region of space between the two nuclei. This simply means that electron density is highest along the axis of the bond. Single covalent bonds that form between nuclei are created from the head-to-head overlap of orbitals and are called sigma (s) bonds. This overlap may involve s-s, s-p, s-d or even p-d orbitals. Another type of bond, a pi (p) bond is formed when two p orbitals overlap. Pi bonds are found in double and triple bond structures.