Theoretical calculations are frequently simplified by setting some of the constants which occur equal to unity. The theory behind this practice is considered. It is shown that if we start with a set of dimensionally homogeneous equations, it is possible, by reducing the set of equations to an equivalent set of dimensionless equations, to assign arbitrary values to any set of dimensionally independent constants occurring in the equations. It is then shown how, after arbitrary mathematical manipulations on the resulting simpler set of equations, the equations can at any stage be restored to dimensional form with the resultant recovery of the suppressed constants.