ratio. The ratio of two quantities expressed in terms of the same unit is the fraction that has the first quantity as numerator and the second as denominator. For example, if in a group of 100 people 5 die, the ratio of deaths to the total number in the group is 5/100=1/20=.05. Ratios are indicated also by writing the two values with a colon between them, e.g., the ratio of 4 to 8 can be expressed by 4:8 as well as by 4/8.

Numerical proportion considered to be an aesthetic ideal in classical design. It refers to the ratio of the base to the height of a rectangle or to the division of a line segment into two in such a way that the ratio of the shorter part to the longer is equal to that of the longer to the whole. It works out to about 1.61803:1. A rectangle constructed from golden sections (segments in this ratio) is called a golden rectangle.

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Degree to which the fuel mixture in an internal-combustion engine is compressed before ignition. It is defined as the volume of the combustion chamber with the piston farthest out divided by the volume with the piston in the full-compression position (see piston and cylinder). A compression ratio of six means that the action of the piston compresses the mixture to one-sixth its original volume. A high ratio promotes efficiency but may cause engine knock.

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