The Gnevyshev–Ohl rule (GO rule) is an empirical rule according to which the sum of Wolf's sunspot numbers in odd cycles with preceding even cycles (E+O) are highly correlated and the correlation is lower if even cycles and preceding odd ones (O+E) are taken (see Figure 1). Sometimes a simplified formulation of the rule is used, according to which the sums over odd cycles exceeds those of the preceding even cycles (see Figure 2). The rule breaks down under certain conditions. In particular, it inverts sign across the Dalton minimum, but can be restored with the "lost cycle" in the end of the 18th century. The nature of the GO rule is still unclear.