Here is a way to find the order of a given sigmatropic rearrangement. The first step is to give numbers to each atom starting with the atoms of the bond being broken as atom 1. Chemists count the atoms in each direction from the broken bond to the atoms that form the new σ-bond in the product. The numbers that correspond to the atoms forming the new bond are then separated by a comma and placed within brackets. This creates the sigmatropic reaction order descriptor.