The first order transition of the ferroelectric and ferroelastic crystal DKDP (KD2PO4) is studied without applied conjugated field, with only the variation of the chemical energy. The shape and the dimensions of the sample, the space and time gradients of temperature determine various situations of phase coexistence: planar phase front or factory roof ones, very dense domain textures or quasi monodomain region in ferro regions, interactions with long range or short ones. The contribution of the phase coexistence to the macroscopic measured dielectric constant is demonstrated. Are also explained the different temperature variations of the dielectric properties during a PF cycle which begin with a “monodomain para” sample, and during a FP cycle which begins with a polydomain sample. This link between microscopic phenomena and macroscopic results enlightens many published results in different crystals.