The phase transitions from the smectic - A phase to the ferroelectric smectic - C*, the antiferroelectric smectic -C*(A) and the incommensurate smectic - C*(alpha) phase are of the three dimensional (3D) XY universality class, where the soft mode condenses either at the edge, at the boundary or at a general point of the smectic Brillouin zone. High-resolution optical polarimetry and linear electro-optic response have been used to determine the temperature dependence of the order parameter in the antiferroelectric liquid crystals MHPOBC, MHP8CBC, EHPOBC, MHP10CBC and 10OTBBB1M7. The conclusions are: (i) In the smectic- A phase antiferroelectric liquid crystals exhibit unusually large tilt fluctuations. (ii) In most antiferroelectric liquid crystals, the critical exponents for tilt fluctuations are neither pure 3D XY nor Gaussian, but somewhat in between (iii) The "reconstructive" phase transitions between incommensurate, ferro-, ferri- and antiferroelectric phases are of first order. (iv) The discrete "clock" Landau model explains well the observed discontinuities. (v) The results of the resonant X-ray scattering experiment and optical polarimetry on 3 and 4-layer "clock" smectic phases are inconsistent.