Chain propagation (sometimes referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a chemical chain reaction. For example, in the chlorination of methane, there is a two-step propagation cycle involving as chain carriers a chlorine atom and a methyl radical which are regenerated alternately:
:Cl + CH → HCl + CH 
:CH + Cl → CHCl + Cl
The two steps add to give the equation for the overall chain reaction:
:CH + Cl → CHCl + HCl.