The conformation of single chains of edgesharing [XA₄] anion-centred tetrahedra is discussed. The conformation can be interpreted as an adaptation of tetrahedral chains to the large halide ions. This adaptation is very reminiscent of that of silicate chains to large cations (Ca, Na, REE). Large anions rotate the [X₂A₆] groups of two edge sharing [XA₄] tetrahedra just as large cations rotate corner sharing [SiO₄] tetrahedra in silicates. As a result of conformation, vierer chains of anion-centred tetrahedra are formed.