Plot of the bond angle α against r(M) for β-M(OH)2 phases and for Ni-, Mg-, Zn- and Co-based LDH phases. The data point for Mg(OH)2 is an average value from six structure determinations: Catti et al. (1995), Černý et al. (1995), Chakoumakos et al. (1997), Desgranges et al. (1996), Isetti (1965), Zigan & Rothbauer (1967). The other data points for β-M(OH)2 phases are from: Kazimirov et al. (2010, Ni(OH)2); Pertlik (1999, Co(OH)2); Parise et al. (2000, Fe(OH)2); Christensen & Ollivier (1972, Mn(OH)2); Bertrand & Dusausoy (1970, Cd(OH)2); Busing & Levy (1957, Ca(OH)2). There are two data points for β-Zn(OH)2: one calculated from Baneyeva & Popova's (1969) structure that is labelled `Zn cBan' and a second from Richardson (2013) that is labelled `Zn c1'. Unfilled circles represent the values for the LDH phases and the dotted line is the linear regression fit for those data (the equation is given in the text).