The two horizontal lines are parallel, and there is a third line that intersects them as shown below.

If we know that the blue angle is `MEASURE` degrees, what is the measure of the orange angle?

`180 - MEASURE`

init({
range: [ [ -1, 11 ], [-1, 4] ]
});
graph.pl = new ParallelLines( 0, 0, 10, 0, 3 );
graph.pl.draw();
graph.pl.drawMarkers(ANCHOR);
graph.pl.drawTransverse( ANCHOR );
graph.pl.drawAngle( KNOWN_INDEX, true );
graph.pl.drawAngle( UNKNOWN_INDEX, false, "#FFA500" );

The pink angles are adjacent to the blue angle and form a straight line, so we know that they measure `180 - MEASURE` degrees.

graph.pl.drawAdjacentAngles( KNOWN_INDEX, true, "#FF00A5" );

It makes sense that the pink angles are equal because they are opposite each other.

One of the pink angles corresponds with the orange angle, and the other pink angle forms an alternate interior angle.

The orange angle equals the pink angles and measures `180 - MEASURE` degrees.

graph.pl.drawAngle( UNKNOWN_INDEX, true, "#FFA500" );

Note that the blue and orange angles are supplementary.