Antiparallel lines

In geometry, two lines ${\displaystyle l_{1}}$ and ${\displaystyle l_{2}}$ are antiparallel with respect to a given line ${\displaystyle m}$ if they each make congruent angles with ${\displaystyle m}$ in opposite senses. More generally, lines ${\displaystyle l_{1}}$ and ${\displaystyle l_{2}}$ are antiparallel with respect to another pair of lines ${\displaystyle m_{1}}$ and ${\displaystyle m_{2}}$ if they are antiparallel with respect to the angle bisector of ${\displaystyle m_{1}}$ and ${\displaystyle m_{2}.}$

In any cyclic quadrilateral, any two opposite sides are antiparallel with respect to the other two sides.