Antiparallel (mathematics)

In geometry, anti-parallel lines can be defined with respect to either lines or angles.

Given two lines ${\displaystyle m_{1}\,}$ and ${\displaystyle m_{2}\,}$, lines ${\displaystyle l_{1}\,}$ and ${\displaystyle l_{2}\,}$ are anti-parallel with respect to ${\displaystyle m_{1}\,}$ and ${\displaystyle m_{2}\,}$ if ${\displaystyle \angle 1=\angle 2\,}$ in Fig.1. If ${\displaystyle l_{1}\,}$ and ${\displaystyle l_{2}\,}$ are anti-parallel with respect to ${\displaystyle m_{1}\,}$ and ${\displaystyle m_{2}\,}$, then ${\displaystyle m_{1}\,}$ and ${\displaystyle m_{2}\,}$ are also anti-parallel with respect to ${\displaystyle l_{1}\,}$ and ${\displaystyle l_{2}\,}$.

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