The Heaviside–Lorentz unit system, like the International System of Quantities upon which the SI system is based, but unlike the CGS-Gaussian system, is rationalized, with the result that there are no factors of 4 π appearing explicitly in Maxwell's equations. That this system is rationalized partly explains its appeal in quantum field theory : the Lagrangian underlying the theory does not have any factors of 4 π when this system is used. Consequently, electromagnetic quantities in the Heaviside–Lorentz system differ by factors of √ 4 π in the definitions of the electric and magnetic fields and of electric charge. It is often used in relativistic calculations, and are used in particle physics. They are particularly convenient when performing calculations in spatial dimensions greater than three such as in string theory.