Metamerism, in chemistry, is used to define the isomeric relationship between compounds with the same polyvalent functional group with heteroatom but differ in the main carbon chain or any of the side chains. It has rather been an obsolete term for isomerism, which has not been recognised by IUPAC in its publications. When Jöns Jacob Berzelius used the term mesomerism in 1831, he did so to describe those substances which possess the same percentage composition but had different properties. What Berzelius implied to be called metamerism is now considered as isomerism.