Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one carbon-to-carbon double bond are called alkenes and those with at least one carbon-to-carbon triple bond are called alkynes. The position of the double or triple bond is shown by a number written either at the start of the name, or just before the -ene or -yne suffix (e.g. pent-2-ene and 2-butyne). The number represents the position of the first of the two carbons making the bond, in the longest carbon chain. Alkenes and alkynes with more than one double or triple bond respectively are named with a prefix preceding the -ene or -yne (e.g. 2,4-pentadiene).