Definition:Compound Ratio

Definition
Let $R_1 = a : b$ and $R_2 = c : d$ be ratios.

The compound ratio of $R_1$ and $R_2$ is defined as:
 * $R_1 R_2 = a c : b d$

That is, it is the product of the two ratios.

Historical Note
Euclid never formally defined a compound ratio, but instead referred to a ratio compounded of two ratios