Parts-per notation
（重定向自Ppbv）
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are ppm (parts-per-million, 10), ppb (parts-per-billion, 10), ppt (parts-per-trillion, 10) and ppq (parts-per-quadrillion, 10).