\begin{definition}[Definition:Harmonic Conjugates/Harmonic Pencil]
Let $AB$ and $PQ$ be line segments on a straight line such that $\tuple {AB, PQ}$ is a '''harmonic range'''.
Let $O$ be a point which is not on the straight line $AB$.
Let $\map O {AB, PQ}$ be the '''harmonic pencil''' formed from $O$ and $\tuple {AB, PQ}$.
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The rays $OP$ and $OQ$ are said to be '''harmonic conjugates with respect to $OA$ and $OB$'''.
\end{definition}