Homologous series are a series of compounds with the same general formula, usually differing by a single methylene (CH₂) unit. The general formula for alkanes is CnH2n+2, for alkenes is CnH2n (with one double bond), and for alkynes is CnH2n-2 (with one triple bond). For n=4, C₄H₈ fits the general formula CnH2n, which corresponds to alkenes. While cycloalkanes also have the formula CnH2n, alkenes are the primary open-chain homologous series with this formula.