Lengthen Command — Use Lengthen to adjust object lengths accurately (intro)

The evolution of CAD editing tools has always been driven by the fundamental need to make precise, non-destructive modifications to geometry after initial placement. In early drafting systems of the 1970s and early 1980s, modifying an object's length typically required deleting the original entity and redrawing it from scratch — a tedious workflow that introduced the possibility of cumulative dimensional errors. As AutoCAD matured through its release cycles, Autodesk recognized that practitioners needed granular control over object dimensions without reconstructing geometry. The LENGTHEN command was introduced to fill a gap between rudimentary editing tools like TRIM and EXTEND, which rely on boundary objects, and the more destructive approach of deleting and re-creating entities.

The central question the LENGTHEN command addresses is straightforward but critical: how does a drafter modify the measured length or included angle of an existing open object — a line, arc, open polyline, or elliptical arc — by a precise amount, without needing to reference another geometric entity as a cutting or extension boundary? This capability distinguishes LENGTHEN from TRIM and EXTEND in a way that is especially relevant for iterative design workflows common in engineering and computer science visualization tasks.

The LENGTHEN command operates on open geometric entities — objects that have distinguishable endpoints, such as lines, arcs, open polylines, splines, and elliptical arcs. It cannot operate on closed objects like circles or closed polylines because those entities have no definable 'end' to extend or shorten. Understanding this constraint is the first principle: LENGTHEN manipulates geometry relative to the endpoint nearest to where you select the object, preserving the opposite endpoint as an anchor.

The diagram above encapsulates the operational semantics of LENGTHEN. In each mode, the pink circle represents the anchored endpoint — the end that remains fixed — while the colored circle at the opposite end shows where the geometry has been adjusted. The critical takeaway from the lower panel is that AutoCAD uses proximity-based selection logic: when you click an object to apply LENGTHEN, the endpoint nearest to your click location is the one that moves. This behavior is deterministic and consistent across all four modes, making it predictable once you internalize the rule. For lines, the modification is purely translational along the line's direction vector; for arcs, the endpoint sweeps along the arc's parent circle, increasing or decreasing the included angle while preserving the center point and radius.

Although LENGTHEN is not a mathematically intensive command from the user's perspective, understanding how AutoCAD internally computes the new endpoint position is valuable for scripting and automation. The underlying geometry depends on the entity type being modified, and the formulas below formalize the relationship between the original object, the selected mode, and the resulting modification.

The new endpoint position for a line is computed by projecting along the line's unit direction vector. If the line runs from point A (the anchor) to point B (the movable end), the direction vector is d̂ = (B − A) / |B − A|, and the new endpoint B' is calculated as B' = A + d̂ × L_new. For arcs, the center and radius remain invariant; only the angular span changes, and the movable endpoint sweeps to a new position on the parent circle. This distinction is why LENGTHEN can specify either a length delta or an angle delta when operating on arcs, and why both sub-options appear in the DELta and Total prompts.

Invoking LENGTHEN can be accomplished by typing LENGTHEN (or the alias LEN) at the command line, by navigating to Modify → Lengthen from the menu bar, or by locating the tool in the Modify panel of the Home tab on the Ribbon. Once invoked, AutoCAD presents a prompt asking you to select an object or choose a mode option. If you select an object without specifying a mode, the command defaults to its measure (query) behavior, displaying the current length and included angle at the command line. Understanding this default is important because many new users expect an immediate modification and are confused when nothing changes.

Consider a scenario where you are drafting a network topology diagram in AutoCAD. A horizontal line representing an Ethernet segment between two nodes currently measures 150 drawing units, but the revised specification calls for a total cable length of 200 units. You want to extend the line from its right endpoint while keeping the left endpoint anchored at the first node's position.

One of the most common points of confusion for intermediate AutoCAD users is knowing when to use LENGTHEN versus the related commands TRIM, EXTEND, and STRETCH. Each command serves a distinct purpose and operates under different constraints, though their effects can sometimes overlap. The table below provides a systematic comparison across the dimensions most relevant to efficient drafting workflows.

For computer science students, the most powerful aspect of LENGTHEN lies not in its interactive use but in its programmability. AutoCAD's AutoLISP and .NET API provide programmatic access to the same geometric modifications that LENGTHEN performs interactively. In AutoLISP, you would use (command "LENGTHEN" "T" 200 (entlast) "") to set the last created entity to a total length of 200 units. In the .NET API, you directly manipulate entity properties — for a Line object, you modify its EndPoint property; for an Arc, you adjust StartAngle and EndAngle. Understanding LENGTHEN's modes helps you reason about which API properties to manipulate.

As you advance into parametric constraints and dynamic blocks, the concept of adjusting an object's length becomes even more powerful. Parametric dimensions allow you to define a length as a variable expression (e.g., cable_length = rack_spacing × 1.15), and changing the variable automatically updates all constrained geometry. Dynamic blocks embed LENGTHEN-like behavior into reusable block definitions with grip-driven stretch actions. These advanced tools build directly on the foundational concepts covered in this lesson.