This is a brief summary of the SI, the modern metric system of measurement. Long the language universally used in science, the SI has become the dominant language of international commerce and trade. Users requiring more detailed information may access SP 811 and SP 330 online from the Bibliography , or order SP 811 for postal delivery . Information regarding the adoption and maintenance of the SI may be found in the section International aspects of the SI . Some useful definitions A quantity in the general sense is a property ascribed to phenomena, bodies, or substances that can be quantified for, or assigned to, a particular phenomenon, body, or substance.
Examples are mass and electric charge.
A quantity in the particular sense is a quantifiable or assignable property ascribed to a particular phenomenon, body, or substance. Examples are the mass of the moon and the electric charge of the proton. A physical quantity is a quantity that can be used in the mathematical equations of science and technology. A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value. The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value.