Lines: Cords in the Unit Disc - without Extremes.

\[
   \overrightarrow{OX}
   =
   (1-t) \overrightarrow{OP}+t~\overrightarrow{OQ}
   =
   \underline{e}_1
   +
   t
   \left(
     \underline{e}_2-\underline{e}_1
   \right),
   0 \lt t \lt 1
\]

<CENTER>
   @Image Klein.Convex.Comb.svg height=100px

</CENTER>

\[
   \overrightarrow{PX}
   =
   t
   \left( 
     \overrightarrow{OQ}-\overrightarrow{OP}
   \right)
   =
   t
   \left(
     \underline{e}_Q-\underline{e}_P
   \right)
\]

\[
   \overrightarrow{QX}
   =
   (t-1)
   \left(
      \overrightarrow{OQ}-\overrightarrow{OP}
   \right)
   =
   (t-1)
   \left(
     \underline{e}_Q-\underline{e}_P
   \right)
\]