The known quarks are listed in Table 1.3 .
Quarks carry quark number, antiquarks being counted negatively.
The net quark number of an isolated system has never been observed to change. V A difficulty with the experimental investigation of quarks is that an isolated quark has never been observed. Quarks are always confined in compound systems which extend over distances of about 1 fm. The most elementary quark systems are baryons which have net quark number three, and mesons which have net quark number zero. In particular, the proton and neutron are baryons.