Home > Draw > Circle: Through 3 Points

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The unique circle passing through three points.

Menu: Draw > Circle: Through 3 Points | Node: Circle: 3 Points | In depth: Circles & arcs

Click three points. There is exactly one circle through any three points that are not in a straight line, so the third click finishes the job with nothing left to decide.

This is the circle for when you know where the edge has to go rather than where the middle is. Three points on a curve you are matching, three corners of a figure you want enclosed: click them and the circle is the answer to that question, not an approximation of it.

Three points in a straight line have no circle through them, so nothing is made. The tool waits rather than inventing something enormous.

Worth knowing, because it surprises people the first time they go looking for one. This circle is stored as its three points, and its middle is worked out from them. There is no point there to snap to, measure from, or build on.

Point: Center of Circle is the command that fixes that: run it on the circle and you get a real center point, constructed, which follows the circle as its three drivers move. A circle you drew from a center already has that point; this kind never does until you ask for it.

All three points, and the circle re-fits whenever any of them moves. Since they sit on the circle, they are also the natural things to build from: they are already exactly where the circle passes, so a segment or a tangent hung off one of them stays attached as the circle changes.

Each click places its point the way the Point tool does, so a 3-point circle can be fitted through existing crossings, and once fitted it keeps up with them.

Three ways to a circle live in the Draw menu, and they differ only in what you know when you start: this one from three points on the rim, Circle: Center Point from a center and a rim point, and Circle: Center Radius from a center and a size borrowed from something else.

< Previous: Circle: Center Radius | Next: Arc: Center, Start, End >

• In the node graph: Circle: 3 Points - build the same circle as a node.
• Point: Center of Circle: the way to get a center point on one of these.
• Circles & arcs: the three definitions and what each is good for.