Point
A point is an exact location. It has no length, width or thickness. We name it with a capital letter and describe where it is with an ordered pair (x, y).
Click an empty spot on the grid to add a point. Drag a point to move it. The dashed path shows how to reach it from the origin.
Line
Two points decide a line. Which part of it you keep gives three different objects.
Drag A and B. Try making the line horizontal or vertical.
Intersections
Where two lines meet is their intersection. Move the points, or jump to a case.
Planes
A plane is a flat surface that goes on forever. Three points that are not on one line fix exactly one plane.
Angles
An angle is the turn between two rays that share an endpoint, the vertex.
Drag either arm around the vertex O.
Using a compass: bisect an angle
The angle demos (bisect and copy) use the angle you set above when it is between 15° and 165°, and 70° otherwise.
Using a protractor: measure an angle
This demo also uses the angle you set above (10° to 170°), and 65° otherwise.
Parallel lines and a transversal
A transversal is a line that crosses other lines. Across two parallel lines it makes eight angles, and certain pairs are always equal or add up to 180°.
The arrow marks on the lines show they are parallel. Tilt the lower line and watch the rules break.
Circles
Drag the labeled points. Lengths are in units, areas in square units.