Explainer · anonymous blocks explained

Anonymous blocks explained: AutoCAD's hidden blocks

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By Sumana Kumar · Published 9 Dec 2022 · Updated 11 Jun 2025

An anonymous block is a block AutoCAD creates and manages on its own, without you naming it. You will never type its name into INSERT, and most of the time you do not even know it exists — yet your drawing may contain dozens of them. They sit quietly in the block table behind the scenes, supporting features like hatches, dimensions, and the variant states of dynamic blocks. The 'anonymous' part means they carry a system-generated, internal name rather than one you chose.

For everyday drafting you can largely ignore anonymous blocks; AutoCAD takes care of them. But they explain a few things that otherwise look mysterious — why a tool like PURGE lists block names beginning with an asterisk, why a dynamic block at a non-default state seems to have a strange name, and why some operations behave oddly on hatches and dimensions. Knowing what anonymous blocks are turns those mysteries into ordinary, expected behaviour.

This page explains what an anonymous block is, the common features that create them, how to recognise them by their naming pattern, and the rare occasions when you actually need to do something about them.

What 'anonymous' means here

Ordinary blocks have names you assign — CHAIR-TASK-600, DOOR-900, your title-block. You create them with BLOCK or WBLOCK and you insert them by name. An anonymous block, by contrast, is created automatically by AutoCAD to implement some feature internally, and it is given a system-generated name you never see in normal use. They are sometimes called 'unnamed' blocks for that reason.

Under the hood they are still real block definitions living in the same block table as your named blocks — they simply belong to AutoCAD's housekeeping rather than to you. You did not make them, you cannot insert them by name through INSERT, and you generally should not try to manage them by hand. They are an implementation detail of how several powerful features are built, exposed only in places like the PURGE list or via the API.

Where anonymous blocks come from

Several common features quietly spawn anonymous blocks. Associative hatches are stored as anonymous block-like entities so the fill updates with its boundary. Dimensions are associative blocks under the hood — each dimension is effectively a small anonymous block bundling its lines, arrowheads and text, which is why exploding a dimension breaks that association. Some object types and third-party objects use them similarly.

The most visible source today is dynamic blocks. When you change a dynamic block to a non-default state — stretch it, pick a visibility variant, flip it — AutoCAD may create an anonymous block to represent that specific configuration internally, while the reference still belongs to your named dynamic block. That is why, if you peek at the block list, you can find anonymous entries tied to dynamic blocks you placed. None of this needs your attention in normal work; it is just how the features are implemented.

Recognising them by name

Anonymous blocks follow a naming convention you can spot. Historically they were named with an asterisk prefix followed by a letter and a number — patterns like *U3, *D5, *A12. The letter hints at the type (for example, U-style names for unnamed/anonymous user blocks, D for dimensions, A and others for various system blocks), and the number just keeps them unique. You will see these names surface in places that list all blocks, such as the PURGE dialog or block-table inspection.

Because the names start with an asterisk and are clearly machine-generated, they are easy to distinguish from your own named blocks. The practical takeaway: if you open a list of blocks and see entries beginning with '*', those are anonymous, system-managed blocks — not something you created or named, and usually not something to delete or rename by hand. They are doing a job for a hatch, a dimension or a dynamic block somewhere in the drawing.

Do you ever need to manage them?

For the most part, no — AutoCAD creates and cleans up anonymous blocks as their host features come and go, and meddling can break the feature they support. Delete the anonymous block behind a dimension and you would damage the dimension; the right move is always to edit or erase the dimension itself, the hatch itself, or the dynamic block reference itself, and let AutoCAD handle the anonymous block underneath.

The one routine place they appear is PURGE. When you purge a drawing of unused named blocks, you may also see anonymous block entries listed; purging removes those that are genuinely unreferenced, helping shrink and tidy a file. That is healthy housekeeping. What you should not do is try to insert, rename or hand-edit anonymous blocks — treat them as read-only internals. If a drawing has an unusual number of them, it usually just reflects lots of hatches, dimensions or dynamic-block states, not a problem to fix.

Anonymous vs named blocks at a glance

The contrast is clean. Named blocks are authored by you, carry meaningful names, are inserted by name with INSERT, and form your reusable library — furniture, doors, symbols, title-blocks. Anonymous blocks are authored by AutoCAD, carry asterisk-prefixed system names, are never inserted by name, and exist only to implement features like hatches, dimensions and dynamic-block states.

Both live in the same block table and both are technically block definitions, but they serve opposite roles: one is your tooling, the other is the program's plumbing. Keeping the distinction clear stops two common confusions — thinking your drawing has 'mystery blocks' someone added (they are just system internals), and worrying that asterisk names in PURGE indicate corruption (they do not). Your job is the named blocks; the anonymous ones look after themselves.

What this means for downloaded blocks

The free blocks you download here are clean, named, static block definitions — furniture, doors, symbols and so on — with no reliance on anonymous internals. Insert one and it becomes a normal named block reference you control. So you will not be wrestling with anonymous blocks because of anything in the download; if any appear in your file, they come from your own hatches, dimensions or any dynamic behaviour you add later.

That said, the moment you start dimensioning a layout that uses these blocks, or hatching the floor around them, AutoCAD will create anonymous blocks behind those dimensions and fills — entirely normally. Recognising them for what they are means you will not be alarmed to see asterisk names in PURGE, and you will know the correct way to remove unwanted ones is to edit the dimension or hatch, then purge, rather than hunting for a phantom block to delete. The named blocks stay yours to manage; the rest is just AutoCAD doing its job.

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Questions

Frequently asked

What is an anonymous block in AutoCAD?+

An anonymous (or unnamed) block is a block AutoCAD creates automatically to implement a feature — like a hatch, a dimension or a dynamic-block state — and gives a system-generated name you never insert by hand. They live in the block table as internal housekeeping.

Why do block names with an asterisk appear in PURGE?+

Asterisk-prefixed names like *U3 or *D5 are anonymous blocks created by AutoCAD for hatches, dimensions and dynamic-block states. Seeing them in PURGE is normal; purging removes the ones that are genuinely unreferenced.

Should I delete anonymous blocks?+

Don't delete them by hand — that can break the dimension, hatch or dynamic block they support. To remove them, edit or erase the host object (the dimension or hatch itself), then run PURGE to clear any that are now unused.

Do downloaded CAD blocks contain anonymous blocks?+

The clean named static blocks you download here don't rely on anonymous internals. Any anonymous blocks in your file come from your own dimensions, hatches or dynamic-block edits — normal behaviour, not a problem with the download.

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