A double bond in chemistry is when two chemical elements are joined together in a chemical bond and share four bonding electrons instead of the usual two. The most common double bond is between two carbon atoms, and can be found in alkenes. There are many types of double bonds, such as in a carbonyl group with a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds (N=N), imines (C=N) and sulfoxides (S=O). In skeletal formula the double bond is shown as two parallel lines (=) between the two joined atoms. When this formula is printed, the equals sign is used.