Arc

The arc command draws a circular arc through three points you click. The arc is computed as the unique circumcircle passing through all three points — no need to specify a center or radius directly. The arc runs from the first click to the third click, passing through the second.

Drawing an arc

1. Type arc in the terminal or click the Arc toolbar button.
2. Click the first point — one end of the arc. Or type X,Y and press Enter for an exact coordinate.
3. Click the second point — a point the arc must pass through (controls curvature and direction). Coordinate entry works here too.
4. Click the third point — the other end of the arc. The arc is placed and the command exits. Coordinate entry works here too.

           ● (2nd click — midpoint on the curve)
          / \
         /   \
        ●     ●
     1st       3rd

A line preview connects the first two clicks while you position the third. From the second click onward a live arc preview tracks the cursor.

Collinear points: if all three points lie on a straight line the arc cannot be computed and no entity is placed. Move the second point off the line and try again.

Other ways to draw an arc

Three points is the default. At some prompts the terminal shows options in square brackets, such as [Center=false] — type the option’s letter to switch it on ([Center=true]) and again to switch it off. Combined, they draw the arc in six more ways:

• In the last step of every method except Start, Center, End, either type a number and press Enter or Space, or move the cursor and click — the cursor is read as that number (a direction for an angle or tangent direction, a distance for a chord length or radius).
• A positive angle runs counter-clockwise from the start, a negative one clockwise. An angle of 0° or whole turns makes no arc and the terminal says so.
• A positive chord length or radius takes the short way round, a negative one the long way. A chord longer than the diameter, or a radius under half the chord, is refused and the terminal states the limit.
• Start, Center, End uses only the direction of the end point from the center: the arc runs counter-clockwise from the start to wherever that direction meets the circle. The tangent direction is the heading, in degrees from the X axis, in which the arc leaves its start.
• To pick the center before the start point, type C at the very first prompt.
• The option letters follow the language of the interface; the letters shown here are the English ones.

Coordinate entry

At any of the three steps you can type an exact position instead of clicking:

1. Type the X value.
2. Press , — the terminal shows [X], [Y{cursor}].
3. Type the Y value.
4. Press Enter to place the point.

Keyboard reference

Grip editing — adjusting endpoints and radius

A selected arc exposes three grips:

Dragging a start or end grip repositions it to the drag location and recalculates both the angle and the radius from that new position relative to the center. The opposite endpoint stays fixed.

Selecting arcs

Supported edit commands

Properties

General

Geometry

Arc vs Circle — when to use which

DXF — ARC entity

Arcs are saved as ARC entities in the DXF file, storing center coordinates, radius, start angle, and end angle. All properties — including color, layer, linetype, linetype scale, and thickness — round-trip without loss. Any DXF-compatible application (LibreCAD, FreeCAD, etc.) reads these as standard arcs.