5.1 Creating Basic Blocks

Blocks are one of the most practical productivity tools in AutoCAD, and the ACU exam tests whether you understand not just how to create one, but why certain choices you make during creation — especially the base point and the layer the geometry sits on — affect how that block behaves every time it gets inserted later.

What a Block Actually Is

A block is a named collection of one or more objects that AutoCAD saves as a single definition inside the current drawing file. Once that definition exists, you can insert as many instances of it as you like, and each instance behaves as a single selectable object no matter how many lines, arcs, circles, or text objects make up the original geometry. This is the same single-object behavior you saw with polylines, but applied to an entire symbol: a door, a bolt, a furniture icon, or a title block logo. Blocks keep repeated symbols consistent across a drawing, because editing the definition (a process called redefining the block) updates every existing instance at once, and they keep file size down because AutoCAD stores the geometry once and simply references it wherever an instance appears.

Creating a Block with the BLOCK Command

You create a new block definition with the BLOCK command (shortcut B), available from the Home tab's Block panel as Create, or by typing BLOCK at the command line. This opens the Block Definition dialog box, which asks for four groups of information:

• Name — a unique identifier for the definition, used later whenever you insert, browse, or search for the block
• Base Point — either click Pick Point to select a point directly in the drawing (typically using an object snap), or type X, Y, and Z coordinates manually
• Objects — click Select Objects to choose the geometry that belongs to the block, or use Quick Select to filter a selection by property
• Behavior and Settings — options such as Annotative, Scale Uniformly, Allow Exploding, a Block Unit dropdown that governs automatic scaling if the destination drawing uses different units, and a Description field for browsing later in DesignCenter

Candidates who prefer working from the command line can type -BLOCK (with the leading hyphen) to step through the same name, base point, and object-selection prompts entirely at the command line, with no dialog box.

The Base Point: The Block's Handle

The base point you specify during BLOCK definition is not just a formality — it becomes the reference location saved permanently with that block definition. Every time you later insert the block, whatever point you click (or coordinate you type) as the insertion point is exactly where that saved base point lands, and the base point is also the pivot AutoCAD uses when you scale or rotate the instance, whether at insertion time or afterward with grip editing.

Because of this, the base point you choose should be a meaningful geometric point on the symbol, not an arbitrary corner of its bounding box. A door block's base point is typically placed at the hinge point, so that inserting it against a wall opening is a simple matter of snapping to the wall's endpoint. A bolt or fastener block's base point usually sits at its center, so it drops neatly onto a hole's center point. Choosing a base point without this kind of intent makes every future insertion harder, since you will constantly need to nudge the instance into place after inserting it rather than snapping it directly where it belongs.

Retain, Convert to Block, or Delete

The Objects section of the Block Definition dialog also asks what should happen to the objects you originally selected, once the new definition is captured:

Convert to Block is the option most drafters use, because it guarantees the drawing still looks exactly the same immediately after defining the block, except that the geometry you selected is now a single block reference instead of loose objects.

Why Layer 0 Matters So Much for Block Geometry

The ACU objectives call out Layer 0 specifically because of a special behavior unique to that layer. If you draw the objects that will become a block while Layer 0 is current, and you leave their color, linetype, and lineweight set to ByLayer (the default appearance for new objects), those objects are considered floating. When you later insert that block into a drawing, each instance adopts the color, linetype, and lineweight of whatever layer is current at the moment of insertion — not the properties of Layer 0 itself.

This is extremely useful for building a reusable symbol library. A single door block drawn on Layer 0 can appear in red on an electrical layer, in black on an architectural layer, and in a dashed linetype on a demolition layer, all from the exact same block definition, simply by inserting it onto different current layers. You never need to maintain separate copies of the same geometry for each color or linetype combination.

Compare this to drawing the same geometry on a named layer, such as Furniture, with ByLayer properties. Every instance of that block will always display using the Furniture layer's color and linetype, no matter what layer is current when you insert it, and no matter what layer you place the instance itself on. If the Furniture layer is later frozen or turned off, every instance you inserted anywhere in the drawing can become invisible along with it — a confusing outcome for a symbol that was supposed to belong to whatever layer the drafter chose at insertion. This is why standard symbol libraries — doors, windows, electrical devices, hardware — are conventionally built on Layer 0 with ByLayer properties, leaving layer control entirely up to whoever inserts the block later.

One exception is worth remembering: Layer 0's floating behavior only works when the objects keep ByLayer color, linetype, and lineweight. If you hard-code an explicit color onto an object while it sits on Layer 0 — overriding ByLayer with, say, a fixed red — that hard-coded value travels with the block wherever it is inserted, regardless of the current layer.

Where the Definition Lives

A block created with BLOCK exists only inside the current drawing file's internal block definition table. It is not automatically available in any other drawing you open. Making that same definition usable elsewhere requires exporting it with WBLOCK, dragging it through DesignCenter, or copying an instance into another open drawing — all covered later in this chapter.

On the Exam

Expect a question describing a block that changes color or linetype depending on where it's inserted, and identifying Layer 0 with ByLayer properties as the reason. Also expect a question distinguishing Convert to Block (replaces the selection with an instance) from Retain (leaves the selection untouched) and Delete (removes the selection), and a question testing whether you understand that the base point becomes the snap point used during every future insertion.