8.1 Trim, Extend & Lengthen

TRIM, EXTEND, and LENGTHEN all reshape existing geometry by adjusting where an object ends, but they solve that problem in two fundamentally different ways. TRIM and EXTEND both measure an object against some other piece of geometry — a cutting edge or a boundary edge — and cut or stretch the selected object until it meets that reference. LENGTHEN, by contrast, needs no reference object whatsoever; it simply changes the selected object's own length by a rule you specify. The ACU AutoCAD exam tests all three commands individually and also tests how they differ from one another, so understanding exactly what each one needs as input is essential.

TRIM and EXTEND Are Mirror-Image Commands

TRIM removes the portion of a selected object that lies past a cutting edge, while EXTEND lengthens a selected object until it reaches a boundary edge. Functionally they are opposites, but they share nearly the same option set and even the same underlying selection logic, which is why AutoCAD lets you flip between them mid-command rather than treating them as unrelated tools.

Quick Mode: Every Object Can Be a Reference

In current AutoCAD, starting TRIM or EXTEND and immediately pressing Enter (without picking specific reference objects first) activates Quick mode, the current default. Quick mode treats every object visible in the drawing as a potential cutting edge (for TRIM) or boundary edge (for EXTEND), so you can start clicking objects to trim or extend right away without a separate reference-selection step. This is faster for simple cleanup work, but in a dense drawing it can catch edges you didn't intend to use.

To restrict the reference set instead, type T for cuTting edges inside TRIM (or B for Boundary edges inside EXTEND) at the first prompt, then select only the specific objects that should act as the cutting or boundary reference before pressing Enter to move on to selecting the objects you actually want to trim or extend. Watch the capitalization in the prompt [cuTting edges/Crossing/mOde/Project/eRase]: the keyword letter for cutting edges is T, not C, because C is already taken by the Crossing selection option. This two-step approach — pick references first, then pick targets — is the classic workflow that Quick mode was added on top of, and the exam may describe either approach depending on how a scenario is framed.

The Shift-Key Toggle

One of the most exam-relevant shortcuts in this pair of commands is the Shift-key toggle. While TRIM is active, holding Shift and clicking an object extends that object to the nearest boundary instead of trimming it — a single-pick override of the command's normal behavior. The same toggle works in the opposite direction: while EXTEND is active, holding Shift and clicking an object trims it back to the nearest cutting edge instead of extending it. This means a drafter cleaning up a set of wall lines can stay inside one command the entire time, using Shift whenever a particular line needs the opposite treatment from the rest.

The Edge Option: Real vs. Implied Intersections

Both TRIM and EXTEND include an Edge option with two settings, and this is where most "why didn't this work" questions on the exam originate. Sometimes the cutting or boundary edge and the object you're trying to trim or extend don't actually cross anywhere on screen — but they would cross if one of them were extended along its own straight or curved path. The default setting, No extend, requires an actual, currently visible intersection; if none exists, AutoCAD reports that no valid edges were found and does nothing. Switching Edge to Extend tells AutoCAD to project the cutting or boundary edge (not the object being trimmed) along its own path to locate this implied intersection, then trim or extend the selected object to that projected point even though the reference edge doesn't visibly reach it. A short line that stops just shy of a wall, for example, can still act as a boundary edge under Extend mode for a line that needs to reach it.

Other Shared Options

Both commands also offer several supporting options worth recognizing by name: Fence lets you draw a temporary selection line across several objects at once, trimming or extending everything the fence line crosses in a single pass. Crossing works similarly using a crossing selection window instead of a fence line. Project controls whether the operation happens in the current UCS plane, the current view plane, or true 3D space — relevant mainly to 3D work and rarely tested for a 2D-focused exam like this one. eRase lets you delete a selected object outright without leaving the TRIM command, and Undo reverses the most recent trim or extend operation without exiting the command entirely.

LENGTHEN: No Reference Edge Required

LENGTHEN solves a different problem: changing how long a single open object is, without comparing it to anything else in the drawing. Because there's no cutting or boundary edge involved, LENGTHEN only works on open objects — lines, open polylines, arcs, elliptical arcs, and splines. It cannot be used on a closed polyline, a circle, or any other object with no free endpoint, since there's nothing for the command to move.

LENGTHEN offers four modes, and distinguishing them precisely is critical for the exam:

Consider a 10-foot line. Selecting DElta and entering 2, then picking near the line's right endpoint, extends that end by 2 feet to a new total of 12 feet; entering -2 instead shortens it to 8 feet. Selecting Percent and entering 150 on that same original 10-foot line stretches it to 15 feet, since 150% of 10 feet is 15 feet. Selecting Total and entering 6 on the same line sets its length directly to 6 feet, regardless of whatever length it currently had — Total ignores the current value entirely and jumps straight to the number you type. The same three modes apply to arcs, except Delta and Total work against the arc's included angle (in degrees) rather than a linear distance, while Percent still scales the arc's current arc length proportionally.

On the Exam

Expect scenario questions that test whether you recognize TRIM/EXTEND's dependence on a reference edge versus LENGTHEN's ability to work on a single object with no reference at all. The Edge option's Extend setting for implied intersections, the Shift-key toggle between TRIM and EXTEND, and the numeric distinction between Delta (increment), Percent (proportional), and Total (absolute) in LENGTHEN are the three details most likely to appear as direct test items.