In the past years, in a series of papers (Antonello & Poretti 1986, Antonello et al. 1990, Mantegazza & Poretti 1992, Poretti 1994) the Fourier decomposition was successfully applied to the light curves of galactic Cepheids with P < 8 d. The separation of these Cepheids into two groups was evident. The first, more numerous (195 stars after the latest inclusion) group is constituted by the classical Cepheids, i.e., the Cepheids which follow the Hertzsprung progression: they occupy a narrow region in the ϕ21 – P plane and are also characterised by a R21 ratio larger than 0.20. The second group, containing 30 objects, is constituted by Cepheids which deviate from the Hertzsprung progression, describing a ‘Z’ crossing the classical sequence in the ϕ21 – P plane; their R21 values are smaller than 0.20. The relation between stars of the upper and lower sequence of the ‘Z’ is confirmed by the ϕ31 – P plane, where these stars describe an unique sequence. As first suggested by Antonello & Poretti (1986), the splitting of Cepheids into two groups can be explained by a different pulsation mode: while the classical Cepheids are pulsating in the fundamental radial mode, the other Cepheids are pulsating in the first overtone radial mode.