A quantity of dimension one is historically known as a dimensionless quantity (a term that is still commonly used); all its dimensional exponents are zero and its dimension symbol is 1 . Such a quantity can be regarded as a derived quantity in the form of the ratio of two quantities of the same dimension. The named dimensionless units " radian " (rad) and " steradian " (sr) are acceptable for distinguishing dimensionless quantities of different kind, respectively plane angle and solid angle.