In the second half of twentieth century, experimental studies of the atomic nuclei revealed the internal structure of protons and neutrons. Proton and neutron consist of a 3 quark combinations.

Two different types of quark called u (up) and d (down) quark are combined.

Quark structure of protons and neutrons: proton consists of 2 up quarks and 1 down quark, while neutron consists of 1 up quark and 2 down quark.