# How to classify the symmetry $C$, $P$ and $T$?

What is the difference between internal symmetries and space-time symmetries? Where would the $C$, $P$ and $T$ symmetries be classified?

On the other hand, internal symmetries refer to transformations acting on internal degrees of freedom of systems, such as fields, charges, etc, which does not transform space-time points. The classic example of internal symmetries are gauge symmetries. In quantum field theory, gauge symmetries are normally given by compact semi-simple Lie groups, such as the QCD's $SU(3)$ and the electroweak's $SU(2)\times U(1)$.