The results of detailed investigations of dielectric constant, ϵb, and optical birefringence in betaine calcium chloride dihydrate are reported. The effect of bias electric field and uniaxial mechanical stress on the temperature range of localization of the incommensurate modulation wave vector k is observed. It is shown that the incommensurate phase consists of several long‐periodic commensurate (LPC) phases. Additional regions of incommensurability wave vector k localization are found. Some of the LPC phases are ferroelectric with spontantaneous polarization arising along a‐ or b‐axis and the other ones are ferroelastic with spontaneous strain Uxy. The temperature region of different LPC phase existence is changed under the influence of a corresponding external field. The observed sequence of LPC phases explains the availability of optical activity within the incommensurate phase.