\begin{definition}[Definition:Compound Ratio]
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.
\end{definition}