The paper demonstrates in an elementary way a geometrical property of the crystal lattice (a dotted homogeneous periodical space). From this property it follows that: 1) the X-ray diffraction by the scattering centres of the space lattice requires, as is well known, that the interference phenomenon look geometrically like a usual reflection from a lattice plane; 2) the concept of equivalent lattice segments, with regard to the metric-interferential effects, can be stated; 3) some immediate relations between Laue's equations and Bragg's formula are derived in a formally rigorous way.