Quadrangle rule

Similarly as the lever rule holds in the two-component systems, in the three- and four-component systems, apply the triangle and quadrangle rules. 

According to the quadrangle rule, every couple of phases, the figurative points of which lie on the ends of one of the diagonals of the general quadrangle (e.g. A, C), can... 

According to the lever rule, phases A and C will first compose phase F and then phase F decomposes according to the lever rule into phases B and D. The quadrangle rule applies especially in phase diagrams of three-component systems with the formation of a congruently melting compound and in the phase diagrams of reciprocal systems. 

From the mixture X component A crystallizes first as well moving the composition of the melt from point X towards point P. After the composition of melt attains point P on the boundary line Pb-Pt, also compound A4B crystallizes, since with regard to the quadrangle rule the melt starts to dissolve compound A according to the reaction... 

The most complicated selection rules - the quadrangle conditions -occur for transitions between configurations where the energy levels of... 

If point X lies in the area of the quadrangle A-A4B-o -e2 (point Xi on Figure 3.32), according to the triangle rule its crystallization path must end in the mechanical mixture of the solid A, A4B, and C (apexes of the triangle A-A4B-C) at the point Pt. Since we are in the crystallization area of component A, at the cooling of the melt crystals, A will appear... 

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