Hello! When an s and a p orbital are hybridised to give and sp orbital, two orbitals are produced. According to my chemistry book, there are two orbitals due to in-phase and out-of-phase combinations of the the s and p orbitals. When we progress to sp2 hybridisation, there are three orbitals, but no textbook I have read explains what happended to the in- and out-of-phase interactions. I played around with different ideas. I know that two p-orbitals on different atoms don't mix when they are not aligned along the same axis because the orbitals have a constructive and destructive interaction - e.g. a px and a py orbital - so I thought that maybe this needed to be considered; otherwise, I don't see why there is not every combination. Take the s, py and pz orbitals, the combinations seem to be: the in-phase interaction of s and py and out-of-phase pz; all in-phase; s and out-of-phase px and in-phase pz; or all out-of-phase. See what I mean? For sp, there is an in-phase or out-of-phase interaction, for the sp2 hybridisation this seems to disappear. Thanks, Nobahar.