Nuclear reaction kinematics are customarily performed in units involving the electronvolt, which derives from accelerator technology. The combination of this practical point with the theoretical relation E = mc makes the unit megaelectronvolt over the speed of light squared (MeV/ c) a convenient form in which to express nuclear mass. However, the numerical values of nuclear masses in MeV/ c are quite large (even the proton mass is ~938.27 MeV/c), while mass excesses range in the tens of MeV/ c. This makes tabulated mass excess less cumbersome for use in calculations. The 1/ c factor is typically omitted when quoting mass excess values in MeV, since the interest is more often energy and not mass; if one wanted units of mass, one would simply change the units from MeV to MeV/ c without altering the numerical value.