# Units and Measurements

Measurement is the most useful form of description in science. Often the most useful measurements are those that have a number and a unit, such as 12.7 inches. Here '12.7' is the number and 'inches' is the unit. This unit of inches in the example is one of the common units in the dimension of length. A number, then, is an expression in numerals. A unit is a recognized way to divide the essence of a dimension for measurement, and a dimension is a measurable physical idea. Here is a bit of advice you can overlook only at your peril: To become fluent in the subject you should memorize the basic background of information. The following units, dimensions, and measures are so basic to the study of chemistry that you could always help yourself by memorizing these. The real test of whether you know this well enough is to recognize the dimensions of any measurement and know its symbol and magnitude from the unit alone.