10.1 Linear, Aligned, Angular, Radius & Diameter Dimensions

AutoCAD's ACU exam objective 4.3.a asks candidates to add five specific dimension types — linear, aligned, angular, radius, and diameter — and, more importantly, to recognize which one is correct for a given geometry. All five dimension types share the same basic anatomy (extension lines, a dimension line, arrowheads or tick marks, and dimension text), and modern AutoCAD lets you place any of them through one unified command, but the underlying measurement logic behind each type differs enough that picking the wrong one produces a dimension that is technically valid yet wrong for the drawing.

The Unified DIM Command

The modern DIM command replaces the need to remember five separate command names for everyday work. Start DIM, then hover the cursor over an object: hovering over a straight line previews a linear dimension, hovering over a diagonal line offers both linear and aligned options, hovering over an arc previews a radius (or, with a command-line option, a jogged radius or arc length dimension), and hovering over a circle previews a diameter dimension. DIM reads the geometry you're pointing at and offers the dimension type that geometry normally takes, but command-line options — Angular, Baseline, Continue, Ordinate, Radius, Diameter, Jogged, aLigned — let you override that guess at any point before you click.

Behind DIM, the five classic explicit commands still exist and still work exactly as they always have: DIMLINEAR, DIMALIGNED, DIMANGULAR, DIMRADIUS, and DIMDIAMETER. Many experienced users, and many exam scenario questions, refer to these by name, so recognizing which explicit command corresponds to which measurement behavior matters even though a single unified command now exists.

Linear vs. Aligned: The Classic Trap

DIMLINEAR measures a purely horizontal or purely vertical distance between two extension line origins — it projects the true distance onto the X-axis or Y-axis, regardless of the actual angle between the two points picked. By default AutoCAD picks whichever orientation (horizontal or vertical) matches the larger projected distance, but a Horizontal, Vertical, or Rotated option can force a specific direction.

DIMALIGNED instead measures the true straight-line distance between the two extension line origins, and its dimension line is automatically drawn parallel to the line connecting those two points, no matter what angle that line sits at.

The trap: pick two endpoints of a diagonal wall or an angled brace with DIMLINEAR, and the resulting number is only the horizontal (or vertical) run — not the actual diagonal length of that wall. Using DIMALIGNED on the exact same two points instead reports the true diagonal length, because the dimension line tilts to match the angle of the feature being measured.

Any time a drawing shows a diagonal edge and the true length of that edge — not its horizontal or vertical component — needs to be called out, Aligned, never Linear, is the correct choice.

Angular Dimensions: Which of Four Angles?

DIMANGULAR measures the angle between two nonparallel lines, the included angle of an arc, or the angle formed by three picked points (a vertex plus two endpoints). After selecting the two lines, AutoCAD does not automatically know which angle to report, because two intersecting lines actually form four angles around their intersection point — two pairs of vertical angles that typically produce two different supplementary values. The angle that gets dimensioned is determined entirely by where the dimension arc is dragged and clicked after the two lines are selected: dragging into a different quadrant around the intersection produces a completely different angle value, even though the same two lines were selected both times.

This makes angular dimensioning a location-sensitive operation in a way linear and aligned dimensioning are not — with linear or aligned dimensions, the measured value is fixed the moment the two points are picked, and only the offset distance of the dimension line changes based on where you drag. With angular dimensions, the drag location changes the actual reported value, since it selects which of the four angles the arc spans.

Radius vs. Diameter: A Drafting Convention, Not Just a Tool Choice

DIMRADIUS and DIMDIAMETER are both valid on any circle or arc — nothing in AutoCAD stops you from placing a diameter dimension on an arc or a radius dimension on a full circle. But standard drafting convention, tested directly on the exam, assigns each to a specific situation:

• Diameter is used for full circles, especially manufactured features like drilled or bored holes, because a diameter value (the distance straight across, through the center) is what a machinist needs to select a drill bit or measure with calipers. Diameter dimension text is automatically prefixed with the diameter symbol (⌀).
• Radius is used for arcs and fillets — a rounded corner, for example, is described by its radius because the arc itself doesn't have a "diameter" the way a full circle does. Radius dimension text is automatically prefixed with R.

Choosing the tool that matches the geometry's real-world function, not simply whichever command technically runs without error, is the actual skill being tested.

Jogged Radius for Off-Page Centers

A very large arc can have a true center point that falls well outside the drawing's visible area or even outside the printable page — drawing a full-length dimension line all the way out to that true center would be impractical. DIMJOGGED solves this by placing a jog (a zigzag break) in the dimension line and letting you specify an alternate, relocated center location point that stands in for the arc's true center, keeping the entire dimension readable within the drawing area while the leading text still correctly reports the arc's true radius value, prefixed with R.

Arc Length Dimensions

A related but distinct tool, DIMARC, measures the actual curved length along an arc — the distance you'd get by tracing along the curve — rather than the radius or a straight chord distance between its endpoints. Arc length dimension text carries a small arc symbol above or before the text specifically so it cannot be confused with a radius or a linear chord measurement at a glance.

Quick Dimension (QDIM)

QDIM generates dimensions for several selected objects in one operation rather than dimensioning them one at a time. After selecting objects, QDIM offers arrangement options at the command line — Continuous, Staggered, Baseline, Ordinate, Radius, and Diameter — and it can also edit an existing group of dimensions it created earlier by re-selecting them. QDIM is a productivity tool built on top of the same underlying dimension types covered above; it doesn't introduce a new kind of measurement.

On the Exam

Expect scenario questions describing a specific geometric situation — a diagonal brace, a drilled hole, a rounded fillet, an oversized arc — and asking which dimension command is correct. The Linear-versus-Aligned distinction on angled geometry, and the Radius-versus-Diameter convention for arcs versus full circles, are the two most heavily tested traps in this section.