The carbon-atom skeletons of aliphatic hydrocarbons may consist of straight or branched chains, or of (non-benzene) rings. In addition, all of the carbon atoms in the skeletons may be joined by sharing single pairs of electrons (a single bond, represented as C:C or C-C), as in the examples above, or there may be some carbon atoms that are joined by sharing two or three pairs of electrons. Such bonds are called double and triple bonds and are represented as C::C or C=C and C:::C or C ≡ C, respectively.
Thus, there can be three kinds of aliphatic hydrocarbons: those whose carbon skeletons contain only single bonds, those that contain some double bonds, and those that contain some triple bonds. These three series of aliphatic hydrocarbons are called alkanes, alkenes, and alkynes, respectively. (There can also be " hybrid" hydrocarbons that contain bonds of two or three kinds.)