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**Varying Gravitational Field - Invariant Tetrahedron??**

Classical Theory of Fields, Landau Lifgarbagez, page 246:

"Strictly speaking, the number of particles should be greater than four. Since we can construct a tetrahedron from any six line segments, we can always, by a suitable definition of the reference system, make a system of four particles form an invariant tetrahedron. A fortiori, we can fix the positions relative to one another in systems of two or three particles."

This is a footnote to a section where the change in reltive spatial distances between bodies in a varying gravitional field is discussed. I have no idea what it means by it. Could someone explain? I have never been so puzzled!!