2.1 Lines, Circles, Rectangles, Arcs, Polygons & Rays

Building the Basic Object Set

Every AutoCAD drawing is assembled from a small set of primitive objects. The ACU AutoCAD exam expects you to know not just that these commands exist, but which option prompt controls size, shape, and orientation — because that is where most real-world (and exam) mistakes happen.

LINE

The LINE command draws one or more straight segments, but each segment is stored as its own independent object (unlike a polyline, covered in the next chapter). After you pick a start point and at least one more point, the command keeps prompting for additional points until you press Enter, Escape, or use one of two special responses:

• Close — available once you've drawn at least two segments; it draws a final segment back to the very first point you picked and ends the command. This is the fastest way to close a triangle or irregular shape without snapping to the start point manually.
• Undo — backs up one segment at a time without leaving the command, useful when a pick point is wrong but you want to keep drawing.

Common trap: Because LINE segments are separate objects, selecting "one side" of a shape drawn with LINE only selects that single segment — not the whole outline. If a question describes needing to move, offset, or fillet an entire multi-segment outline as one object, that is a signal for PLINE, not LINE.

CIRCLE

The default CIRCLE workflow is Center, Radius — pick a center point, then type or pick a radius. But the command exposes several other construction methods, each solving a different real-world layout problem:

Common trap: Mixing up radius and diameter. If a problem states a 10" diameter hole, typing "10" at a default Center-Radius prompt produces a 20"-diameter circle. Always confirm which value the prompt is asking for, or explicitly invoke the Diameter option.

RECTANG

RECTANG draws a rectangle as a single closed polyline object from two diagonal corner picks. Before picking the first corner, you can set modifying options that persist for subsequent rectangles until changed:

• Chamfer — cuts angled corners of a specified distance (or two distances)
• Fillet — rounds all four corners to a specified radius
• Width — sets a uniform line width for the polyline's segments
• Elevation / Thickness — 3D properties, rarely tested at ACU level
• Area and Rotation — alternate ways to size and orient the rectangle instead of picking a second corner directly

Common trap: Chamfer and Fillet settings stick between rectangles. If a previous rectangle used a 0.5" fillet, the next RECTANG will silently apply that same fillet unless you reset it to zero.

ARC

ARC is the most option-heavy basic shape command. The default invocation (just clicking three points) uses 3-Point, but the full option set — accessed by starting with a letter option or via the ribbon's Arc gallery — includes combinations of Start, Center, End, Angle, Direction, Radius, and Length of Chord:

• 3-Point — pick a start, a point on the arc, and an end point
• Start, Center, End — pick start, then center, then an end angle (radius is implied by the start-to-center distance)
• Start, Center, Angle — like above, but you type an included angle instead of picking an end
• Start, Center, Length — you specify the chord length instead of an angle
• Start, End, Radius — useful when you know two endpoints and a radius but not the center
• Start, End, Angle or Start, End, Direction — useful when the arc must leave the start point tangent to something
• Center, Start, End — same geometry as Start-Center-End, entered in a different order

Common trap: AutoCAD always sweeps arcs counterclockwise from the start point by default. If you pick a start and end point expecting a short arc but get the long way around the circle instead, you likely need to swap your point order or specify a negative angle — the software isn't broken, it's following the CCW convention.

POLYGON

POLYGON draws a regular polygon (3 to 1024 equal sides) as a single closed polyline. After specifying the number of sides and a center point, you choose one of two fitting methods:

• Inscribed in circle — you specify a radius, and the polygon's vertices sit exactly on a circle of that radius (the polygon is smaller than the circle)
• Circumscribed about circle — you specify a radius, and the polygon's edge midpoints are tangent to a circle of that radius (the polygon is larger than the circle)

Alternatively, the Edge option skips the center/radius approach entirely — you specify two points that define one edge, and AutoCAD builds the rest of the polygon from that edge length and orientation.

Common trap: For an identical radius value, Inscribed and Circumscribed do not produce the same-size polygon. Circumscribed always produces a larger polygon than Inscribed for the same radius, because the radius is measured to different reference points (vertex vs. edge midpoint).

RAY and XLINE (Construction Line)

When you need infinite reference geometry for aligning other objects rather than a finished, plottable shape, AutoCAD provides two dedicated tools:

• RAY — starts at a specified point and extends to infinity in one direction only
• XLINE (Construction Line) — passes through a specified point and extends to infinity in both directions

Both are commonly used as temporary layout aids (e.g., projecting alignment points between views) and are typically placed on a dedicated non-plotting or construction layer, then erased once they've served their purpose. Because they have no defined endpoints, standard length-based editing (like LENGTHEN) doesn't apply to them the way it does to lines and arcs.