We use natural units to make things simpler for ourselves when doing tricky sums. Two constants come up all the time in the equations of particle physics:

~~The reduced Planck constant, ‘h-bar’ = 1.05 x 10^{-34} kg m^{2} / s~~- The speed of light, c = 3 x 10^{8} m / s

We can choose to express these constants in any units we wish, for example:

1. h-bar = 1