ABSTRACT
12.1 The difference between left and right congruence forces us to make a choice each time we consider such congruence relations. Normally people do not really like to make choices all the time, so the situatiuon where the choices have been made irrelevant is very often called “normal” in mathematics (also in dictatorial regimes, but there is no relation with mathematics). Normal subgroups, apart from making the left congruence relation with respect to it equivalent to its right version, are very useful because the formation of quotient group is now possible i.e. the set of classes now carries a natural group structure (see Proposition 12.12).
