Heat can cause the double bonds to form a ring. Thermal cycloadditions usually have (4n + 2) π electrons participating in the starting material, for some whole number "n". Because of orbital symmetry, most cycloadditions are suprafacial-suprafacial. Rarely, they are antarafacial-antarafacial. There are a few examples of thermal cycloadditions which have 4n π electrons (for example the [2 + 2] cycloaddition). These proceed in a suprafacial-antarafacial sense. For example, the dimerisation of ketene has a orthogonal set of p orbitals. Those p orbitals allow the reaction to proceed using a crossed transition state.