Dual spark plugs are occasionally employed on certain modem S.I. engines operating with very lean fuel-air mixtures, or with larger proportion of EGR, or on engines operating in other conditions which are difficult to ignite. Different researchers have advanced different reasons for gains due to dual spark plugs; namely, shorter flame travel distance, change in flame speeds and shapes, etc. According to the hypothesis proposed by the present author, the exact cause of the gains due the dual plug system is different from that put forth by the other researchers. The hypothesis states that, in adverse engine operating conditions when the success rate of spark ignition is poor, the presence of two spark plugs increases the probability of generation of the self-sustaining flame kernel thereby reducing the probability of misfires. The hypothesis is proved by conducting a brief parametric study using a validated mathematical model. Furthermore, the hypothesis forms the basis of the explanations to the behaviour of certain dual plug engines resulting in a consistent and acceptable description of the engine behaviour.