The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix of the alkane with -ene. As per the IUPAC rules, the longest carbon chain containing the maximum number of double bonds is identified as the parent chain and is numbered such that the doubly bonded carbon atoms receive the lowest possible numbers. The location of the double bond is indicated by the number of its first carbon atom. In branched alkenes, the preference of numbering is given to the double bond over the substituents. The substituents are numbered according to their position in the parent chain and are listed alphabetically. However, the name and position of the substituent groups are cited before the double bond. In cycloalkenes, the ring is considered the parent chain, and the lowest possible numbers are assigned to the double bond. Alkenes having multiple double bonds in the parent chain are termed polyenes and have the infixes -adien- and -atrien- for two and three double bonds, respectively. Some smaller alkenes have IUPAC-acknowledged common names. For example, ethene is commonly known as ethylene, propene is known as propylene, and so on.