3.1 Drawing Open & Closed Polylines

The PLINE command is one of the most heavily tested tools in the Draw and Modify Objects domain, and it introduces a concept that trips up new AutoCAD users: a single object made of multiple connected segments. Unlike LINE, which creates a brand-new independent entity every time you click a point, PLINE keeps every segment you draw — straight or curved — bundled into one polyline object until you end the command.

PLINE vs. LINE and ARC: Why the Distinction Matters

When you draw a five-sided shape with LINE, you get five separate line entities. Select one edge and only that edge highlights; the other four stay put. Draw the same shape with PLINE and selecting any edge highlights the entire outline, because AutoCAD stores it as one object with a list of vertices. This single-object behavior is the entire reason PLINE exists as a distinct command rather than a variation of LINE.

The same logic applies to arcs. ARC creates one arc entity per use. A PLINE that includes curved segments still behaves as one object, mixing straight and curved geometry under a single selection handle, a single set of properties, and a single entry in the Properties palette.

Command-Line Options While Drawing a Polyline

After you start PLINE and specify a start point, AutoCAD's command line displays a running set of options you can invoke by typing the capitalized letter shown in brackets:

Once you switch into Arc mode, a parallel set of options appears (Angle, CEnter, Direction, Line, Radius, Second pt, and more) that mirror the flexibility of the standalone ARC command, but every arc you draw still belongs to the same polyline object. Typing Line while in Arc mode switches back to straight segments — you can toggle between the two as many times as the drawing requires.

Width: Uniform, Tapered, and the Halfwidth Trap

The Width option prompts for a starting width and an ending width separately. Entering the same value for both produces a uniform-width segment; entering different values produces a tapered segment that is wide at one end and narrow at the other — the technique used to draw arrowheads, leader shafts, and schematic wall thicknesses without a separate hatch.

Halfwidth works identically but asks for half the total width, measured from the polyline's centerline to one edge. A common exam trap is assuming Halfwidth and Width take the same input: entering "3" for Halfwidth produces a segment that is 6 units wide overall, not 3. Once you set a width, it carries forward to every new segment until you change it again — a single Width entry does not automatically reset to 0 for the next segment.

Close: A Command Option, Not Just a Click Back to Start

Typing C (or selecting Close from the dynamic input menu) at any point after drawing at least two segments draws a straight closing segment back to the polyline's original start point and marks the polyline's Closed property as Yes. This distinction matters because you could, instead, manually move the crosshairs back to the same coordinates as the start point and click — visually the shape looks identical, but the polyline's Closed flag stays No, because AutoCAD only sets that flag through the Close option (or later through PEDIT). A polyline that only looks closed but is not flagged as closed can behave differently in commands that check the Closed property, such as hatch boundary detection or the AREA command's handling of open shapes.

Choosing PLINE Over Separate LINE and ARC Objects

The ACU objectives specifically call out drawing polylines as a core skill because real drafting work rarely calls for isolated segments. Reach for PLINE instead of separate LINE/ARC objects when:

• You need to select, move, or offset an entire outline as one operation — offsetting a polyline produces a clean, uniformly spaced closed or open shape in a single click, while offsetting five separate lines requires five separate offset operations that will not automatically join at the corners.
• The shape will become a hatch or fill boundary — HATCH performs best against a single closed polyline rather than several individual objects that must all touch perfectly.
• You need varying or tapered width, such as a leader shaft, a schematic pipe run, or an arrow — LINE has no width property at all.
• You want accurate area and perimeter data from a single object using the Properties palette or the MEASUREGEOM command, rather than summing multiple lengths by hand.
• The geometry will later be extruded or revolved in a 3D workflow — a closed polyline profile extrudes cleanly into a solid, while a set of unjoined lines and arcs typically will not.

On the Exam

Expect the ACU exam to test the PLINE Width/Halfwidth relationship, the difference between the Close option and manually snapping back to a start point, and scenario questions asking which drafting situation calls for a polyline instead of individual LINE and ARC objects. If a question describes selecting an entire multi-segment outline with one click, or applying one width to an entire tapered shape, the expected answer is almost always PLINE.