X-ray diffraction measurements have been made of the wavevector and intensity of satellite diffraction spots as a function of temperature in two examples of crystal B phases. In N-(5-n butyloxybenzylidene)-4-n-heptylaniline (5O.7) a simple sinusoidal layer modulation of fixed wavelength grows in amplitude with decreasing temperature until the B-phase becomes unstable at the first-order B-to-G phase transition. In 4-n-pentylbenzenethio 4'-n-duodecyloxybenzoate (12S5) the layer modulation approximates to a ramp function which grows in wavelength at constant ramp angle with decreasing temperature until the G phase, having a very similar tilt angle is formed at a first-order transition.