Résumé
Three classes in the family of aperiodic crystal systems are the displacive modulated phases, the incommensurate composites and quasicrystals. In each of these families one can have a phase transition incommensurate-incommensurate within the incommensurate phase. This transition can be described as a transition from a smooth dependence on the parameters to a discontinuous one. This transition usually is accompanied by the occurrence of a soft excitation that can be considered as a soft phason. The character of the phason and the phase transition is discussed for an example from each class. Finally the situation in a more complicated system with two different subsystems is considered: the case of coupling of a ferroelectric and antiferromagnetic phase.