Each meson has a corresponding antiparticle (antimeson) where quarks are replaced by their corresponding antiquarks and vice versa. For example, a positive pion (π) is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion (π), is made of one up antiquark and one down quark. Although tetraquarks with two quarks and two antiquarks can be considered mesons they are not listed here.