In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as B = 1 3 ( n q − − n q ¯ ¯ ) , where ⁠ n q ⁠ is the number of quarks, and ⁠ n q ¯ ¯ ⁠ is the number of antiquarks. Baryons (three quarks) have a baryon number of +1, mesons (one quark, one antiquark) have a baryon number of 0, and antibaryons (three antiquarks) have a baryon number of −1. Exotic hadrons like pentaquarks (four quarks, one antiquark) and tetraquarks (two quarks, two antiquarks) are also classified as baryons and mesons depending on their baryon number.