Difference between positional isomerism and metamerism (organic chemistry)
If functional group carbon is part of principal chain then metamers are also positional isomers. In case of polyvalent functional group like sulphoxide($\ce{&gt;S=O}$), secondary amine ($\ce{-NH-}$) etc. structure formed by change in structure of alkyl groups will be metamers only.  For example, diethyl amine ($\ce{Et2NH}$) and methyl propyl amine ($\ce{MeNHPr}$) are metamers.