Monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally) hybridized atoms that contain $\ce{(4n + 2) \uppi\!\mbox{-}electrons}$ (where $n$ is a non-negative integer) will exhibit aromatic character. The rule is generally limited to $n = 0 – 5$. This rule is derived from the Hückel MO calculation on planar monocyclic conjugated hydrocarbons $\ce{(CH)_{m}}$ where $m$ is an integer equal to or greater than 3 according to which $\ce{(4n + 2) \uppi\!\mbox{-}electrons}$ are contained in a closed-shell system. Examples of systems that obey the Hückel rule include: Systems containing $\ce{4n\;\uppi\!\mbox{-}electrons}$ (such as cyclobutadiene and the cyclopentadienyl cation) are 'antiaromatic'.