\begin{definition}[Definition:Conjugate Lines]
Let $\CC$ be a circle.
Let $\PP$ and $\QQ$ be the straight lines in the plane of $\CC$.
Let $P$ and $Q$ be the poles of $\PP$ and $\QQ$ with respect to $\CC$ respectively.
Let $P$ and $Q$ be such that $P$ lies on $\QQ$ and $Q$ lies on $\PP$.
Then $\PP$ and $\QQ$ are known as '''conjugate lines with respect to $\CC$'''.
\end{definition}