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.

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

Key	Action
0–9, ., -	Start X coordinate entry
,	Lock X and move to Y entry
Backspace	Delete last typed character
Enter	Confirm typed coordinate
Escape	Discard all placed points and exit

Grip editing — adjusting endpoints and radius

A selected arc exposes three grips:

Grip	Position	What it does
Center	Geometric center of the circumcircle	Moves the whole arc; radius and angles unchanged
Start	First endpoint on the arc	Drag to move the start along the circumcircle — changes both start angle and radius
End	Last endpoint on the arc	Drag to move the end along the circumcircle — changes both end angle and radius

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

Method	Behaviour
Click	Selects if the click lands near the arc curve (not the chord)
Drag right (strict)	Sample points distributed along the arc must all lie inside the box
Drag left (crossing)	Any sample point on the arc that falls inside the box selects it

Supported edit commands

Command	What happens to the arc
Move	Translates the center; radius and angles unchanged
Copy	Creates an identical arc at a new position
Rotate	Rotates the center and shifts start/end angles by the rotation amount
Mirror	Reflects the center and inverts start/end angles across the mirror axis
Scale	Scales the center position and multiplies the radius by the scale factor
Offset	Creates a concentric arc at a larger or smaller radius, same angle span
Delete	Removes the arc

Properties

General

Property	Default	Meaning
Color	256 (ByLayer)	ACI color index
Layer	0	Layer assignment
Linetype	ByLayer	Named linetype pattern
Linetype Scale	1	Scale factor on the linetype pattern
Thickness	0	Extrusion thickness

Geometry

Property	Meaning
Center X / Center Y	Center of the circumcircle
Radius	Radius of the circumcircle
Start Angle	Angle in degrees where the arc begins (measured from the positive X axis)
End Angle	Angle in degrees where the arc ends

Arc vs Circle — when to use which

	Arc	Circle
Span	Partial — first to third click	Full 360°
Input method	Three points on the curve	Center + radius (click or type)
Typed input	X,Y coordinate for each point	Radius value (center also accepts X,Y)
Resize after placement	Drag start/end grips	Drag any cardinal grip
Best for	Fillets, rounded corners, arched paths	Full holes, round features

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 (AutoCAD, LibreCAD, etc.) reads these as standard arcs.