Quark-gluon separation in three-jet events

We discuss the necessity of a separation of quarks and gluons in $q\overline{q}g$ three-jet events in $e^{+}{e}^{-}$ annihilation in order to obtain quantitative tests of quantum chromodynamics. The possibility of such a separation is investigated with special emphasis on the case that one or two of the jets are identified as quark jets by a semileptonic decay of a $c$ or $b$ quark. It is shown in detail how gluon energy spectra and gluon-jet properties can be deduced from these measurements.