Abstract: The low-symmetry forms of the clinopyroxene structure fit the rules proposed previously (Chisholm, 1981) but the low-symmetry forms of ortho- and proto-pyroxene do not. The rule governing the packing of the tetrahedral chains into layers is therefore revised to take into account the different chain configurations. Starting with simple sequences of. and × chains (Thompson, Hallimond Lecture 1979, unpubl.) a scheme is proposed for deriving most of the known pyribole structure types and their space groups.