Blocks in AutoCAD are clusters of objects gathered together, given a name, and reused as often as you like. Autodesk sums up the command that makes one in a single line on its BLOCK page — "Creates a block definition from selected objects".
What makes a block worth the trouble is not the tidying up. It is that one definition feeds many insertions: change the definition once and every insertion follows, however scattered they are through the set. To learn modelling and documentation end to end, see the AutoCAD for construction course.
What blocks in AutoCAD are, and how they differ from groups
A block is a definition stored in the drawing; what you see on screen is an insertion of that definition. The three doors in the image at the top of this article are three insertions of one definition, and editing the definition changes all three.
Beginners routinely confuse blocks with groups, since both gather objects together.
The question that settles it is short: does this cluster repeat anywhere else? Doors, sanitary ware, grid bubbles and title blocks all repeat, so they become blocks. The notes belonging to one room resemble no other room's notes, so a block gains nothing there and a group is the right tool.
Creating a block with the BLOCK command
Type B and Enter, and the Block Definition dialog opens. This dialog decides whether the block will be pleasant to use later on.
The numbers mark these four areas.
| No. | Element | What it is |
|---|---|---|
| 1 | The Name field | Autodesk records that the name takes up to 255 characters, including letters, numbers, spaces and any special character not reserved by the operating system |
| 2 | The Base point area | The insertion point, defaulting to 0,0,0. Pick point closes the dialog briefly so you can pick it in the drawing |
| 3 | The Objects area | Selects the geometry that goes into the block, with three ways of treating the originals |
| 4 | The Behavior area | Annotative, Scale uniformly to prevent non-uniform scaling, and Allow exploding |
The three Objects options decide whether the original geometry survives, and the Block Definition dialog reference describes each of them plainly.
| Option | Autodesk's wording | Which means |
|---|---|---|
Retain | "Retains the selected objects as distinct objects in the drawing" | The originals stay, still as loose objects |
Convert to block | "Converts the selected objects to a block instance in the drawing" | The originals become an insertion of the block you just made |
Delete | "Deletes the selected objects from the drawing" | The originals go, leaving only the definition inside the drawing |
Allow exploding is ticked by default, and Autodesk describes it as "Specifies whether or not the block reference can be exploded". Clearing it makes the block unbreakable, which suits title blocks and standard symbols nobody should be editing by hand.
The insertion point, and naming blocks
The insertion point is where the block hangs from your cursor. Choose it carelessly and every insertion needs moving afterwards, which wastes the whole point of a block.
Put the insertion point somewhere that means something for that particular component:
| Kind of block | Where the insertion point belongs |
|---|---|
| Door leaf | The hinge. Snap to the edge of the opening and the leaf lands right |
| Window | One end of the opening, on the same side as the width in its name |
| Sanitary fitting | The centreline of the fitting, at the face against the wall |
| Grid bubble | The centre of the circle |
| Level marker | The arrow tip, the point that touches the surface being labelled |
| Title block | The bottom right corner of the sheet border |
A block name should say three things: what the component is, how big it is, and which variant. Names like CD-900 for a single door leaf 900 wide, CD2-1600 for a double door, CS-1200x1400 for a window, TB-BONCAU for a fitting, KT-A3 for an A3 title block. The name is what counting commands work from, so a naming rule from the start is what makes scheduling possible later.
Inserting a block: the Blocks palette
The INSERT command, aliased I, does not open a dialog in the 2027 release as it did in older versions. The INSERT page describes it as "Displays the Blocks palette, which you can use to insert blocks and drawings into the current drawing".
It carries four tabs: Current Drawing for blocks already defined here, Recent for ones used lately, Favorites, and Libraries for blocks pulled from another drawing file. Its advantage over the old dialog is that it stays open all day: finish placing one block and pick the next without recalling the command.
Three of the Options are worth knowing. Insertion Point is ticked so you pick the spot with the mouse. Repeat Placement keeps placing copies. Explode breaks the block apart as it lands — rarely what you want.
The handiest trick lives in the scale factor. Run the command-line form -INSERT and give an X scale of minus 1, and the block mirrors: a left-hand door out of a right-hand door block, with no second block to build or maintain. This is behaviour observed in the 2027 release; the command's help page says nothing about negative scale factors.
One thing Autodesk does state, and it matters when drawings arrive from elsewhere: "Inserting a drawing file as a block also imports all of its block definitions into the current drawing". Insert one DWG and you inherit every block definition it carries, which is why received drawings so often swell with unfamiliar blocks.
Why block geometry goes on layer 0
This is the detail that decides whether a block fits into the drawing's layer system, and it is the one all three Vietnamese tutorials I read left out.
The rule fits in one sentence: draw the block's geometry on layer 0, then define the block. Geometry drawn on layer 0 takes the colour and linetype of whichever layer the block is inserted onto. Inspect such an insertion and its Layer is the destination layer while its colour reads ByLayer.
Draw on any other layer and the opposite happens: the block keeps the colour it was built with, and moving it to another layer will not change that. A block library built the wrong way here is a library that cannot be shared between projects with different layer standards.
WBLOCK: taking a block to another drawing
A block made with BLOCK lives only in the current drawing. To use it elsewhere you have to write it out to its own DWG file, and WBLOCK is the command for that. Autodesk describes it as "Saves selected objects or converts a block to a specified drawing file". The Write Block dialog reference lists its sources.
The three sources, in Autodesk's own words:
| Source | Autodesk's wording | Use it when |
|---|---|---|
Block | "Specifies an existing block to save as a file" | The block already exists and belongs in a library |
Entire Drawing | "Selects current drawing to save as another file" | Writing the whole drawing out as a new file |
Objects | "Selects objects to save as a file" | Picking geometry directly, with no block defined first |
Entire Drawing has a useful side effect known from production work: it is a way of cleaning up a drawing that has grown inexplicably heavy, because the file written out holds only what is genuinely in use.
The Insert units field at the bottom decides how the block is rescaled when it lands in a drawing using other units. Autodesk describes it as "the unit value to be used for automatic scaling when the new file is dragged from DesignCenter or inserted as a block in a drawing that uses different units".
Exploding, editing and counting blocks
All three concern blocks, and each has its own command.
EXPLODE, aliased X, breaks one insertion back into loose objects. The definition stays in the drawing and other insertions are untouched. A block whose Allow exploding box was cleared refuses.
Editing a block can go two ways. By hand: explode one insertion, edit the geometry, then call BLOCK again with exactly the old name; the software asks Redefine it?, and answering Yes updates every insertion. The tidier way is the block editor, BEDIT, which opens its own grey workspace with BSAVE and BCLOSE to finish.
For counting, the 2027 release has a COUNT command of its own that tallies either the whole drawing or an area you outline, and can drop the result into the drawing as a table.
Editing and counting are both big enough subjects to deserve their own articles, so this one stops at an introduction.
Frequently asked questions
Which command creates a block?
BLOCK, aliased B. The Block Definition dialog asks for three things: a name, a base point, and the objects to include.
How do I insert a block I have already made?
Type I to open the Blocks palette and pick it from the Current Drawing tab. Blocks held in another file live under the Libraries tab.
Why does my block come out the wrong colour?
Most likely its geometry was drawn on some layer other than 0. Only geometry drawn on layer 0 takes the colour and linetype of the layer it is inserted onto.
What is the difference between BLOCK and WBLOCK?
BLOCK creates a definition that lives only in the current drawing. WBLOCK writes a block, or any selected objects, out to a separate DWG file for use elsewhere.
Is there a catch to inserting a whole DWG file?
One worth knowing: Autodesk states that doing so imports every block definition from that file into the current drawing.
How is a block different from a group?
A block has its own definition, so it can be copied, counted and carried to another drawing. A group is only a way of gathering objects for selection; it copies nothing and counts as nothing.
Related articles
- Layers in AutoCAD — a standard layer set, locked layers and deleting layers
- How to draw a rectangle in AutoCAD — the REC command, eight options and the settings that persist
- How to enter coordinates in AutoCAD — four ways to type a point, the comma trap and the # prefix
- What is AutoCAD — a handbook for people working in construction