In the ethane Lewis formula shown above all bonds are represented as single lines called single bonds. Each single bond is made up of two electrons, called bonding electrons. It is also possible for two atoms bonded together to share 4 electrons. This bonding pattern is represented by two lines, each representing two electrons, and is called a double bond. The ethylene molecule shown below is an example. Finally, sharing of 6 electrons between two atoms is also possible. In such case, the representation uses three single lines, an arrangement called a triple bond. The acetylene molecule provides an example of a triple bond.
This terminology (single, double, or triple bond) is very loose and informal. The formulas shown above do not do justice to the actual nature of the bonds. All they do is show how many electrons are being shared between the two atoms (2, 4, or 6) but they say nothing about the electronic distribution, or the relative energies of the bonds, or the types of orbitals involved. They are, however, very useful in many situations.