In Chapter 3 we discussed common fractions. This chapter deals with decimal fractions, commonly called decimals. Numbers that fall between any two whole numbers, such as 0.42 and 17.001, are decimal fractions. Whole numbers are also decimal fractions; their decimal places contain zeros, which are often not shown, unless they are needed to imply a certain degree of accuracy in a measurement. For example, 3.00 cm implies greater accuracy than simply 3 cm written without the zeros.

Notice that a whole number is both a decimal fraction and an improper fraction whose denominator is one. For example, the whole number 5 can be thought of as 5.0 or 5/1.

The use of decimals in machine details and assembly drawings came into wide use during World War I. The automobile, aircraft, and machine tool industries pioneered the use of the decimal system in American manufacturing. Some industries, for example, the building trades, still give dimensions in common fractions but these fractions usually cannot communicate the accuracy needed in tooling and machine use. Decimal fractions are necessary for these jobs and in the majority of science and engineering computations.