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I don't want the definition of positional isomerism and metamerism but want what differentiates the two types of isomerism.
I know metamers have different alkyl groups on either side of it, but doesn't that just mean the position of the functional is changing? Hence shouldn't it be a positional isomer.
If functional group carbon is part of principal chain then metamers are also positional isomers. In case of polyvalent functional group like sulphoxide($\ce{>S=O}$), 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.
Actually metamerism comes under positional isomerism. It's like this "metamerism is a subset of positional isomerism". When a compound has a functional group as a part of its chain then we call positional isomerism as metamerism to specify that the functional group is a part of the chain.
