Parallel, perpendicular, intersecting — and how angles pair up on a line and at a point
Drag the slider to change the crossing angle. Watch the four angles — opposite ones stay equal.
Angle a
50°
Angle b
130°
Vertically opposite angles are equal: a = a and b = b.
Angles side by side make a straight line: a + b = 180°. (50 + 130 = 180)
All four angles go around the point: a + b + a + b = 360°. (50 + 130 + 50 + 130 = 360)
Line type now: intersecting