α-sulphur and β-sulphur  are dynamic allotropes. At certain temperature called transition temperature they coexist. Transition temperature in this dynamic allotropy is

Both α and β forms of  sulphur coexist at 369KThere exists a dynamic equilibrium between them.This is called enantiotropy (or) Dynamic allotropyBelow 369K , α-sulphur(rhombic) is stableIt is thermodynamically most stable form of sulphurAbove 369K , β- sulphur(monoclinic) is stable