Dear Forum, I am trying to learn more about the electroproduction of rho meson off proton. e + p -> e + p + rho One of the commonly used kinematic variable is tprime, defined as t-tmin. The definition I could find for tmin is that it is the minimal value of t kinematically necessary to put rho and proton system on shell. However I still do not know how it is calculated and what does 'putting the system on shell' mean. More explanation will be greatly appreciated. Thanks for any response you might have.