Illustration of the internal structure of a proton. A proton is a subatomic particle with a positive electrical charge, found inside the nucleus of an atom alongside neutrons. Protons are made up of smaller particles called quarks, which come in six different 'flavours'. In a proton, there are two 'up' quarks and one 'down' quark, represented here by the letters D and U. Subatomic particles made up of quarks are generally termed hadrons. In addition, hadrons made of odd numbers of quarks, like protons and neutrons, are called baryons. The quarks within hadrons are held together by the strong nuclear force, carried by the so-called gluon - here represented by helices.