1. Drawing Objects (Graphical Entities)

These are the items that create the visible drawing.

Examples:

Lines
Polylines
Arcs
Circles
Splines
Ellipses
Hatches
Text and MText
Dimensions
Leaders
Tables
Blocks (inserted references)
3D solids
Surfaces
Meshes
Point objects

Each object has:

A unique object ID (handle)
A type definition
Geometric data
Object properties
Relationships to other database objects

Example:

A LINE object contains:

Start point coordinates
End point coordinates
Layer assignment
Colour setting
Linetype
Linetype scale
Lineweight
Transparency
Plot style information

2. Object Properties

Properties control how objects behave and appear.

They may be:

A. Object-specific properties

Stored directly with the object.

Examples:

Circle radius
Arc start/end angles
Text height
Dimension style reference
Solid geometry

B. General object properties

Common to many object types:

Layer
Colour
Linetype
Linetype scale
Lineweight
Transparency
Material
Plot style
Thickness
Visibility

These can be assigned:

BYLAYER — inherited from the object’s layer
BYBLOCK — inherited from the block reference
Explicit value — stored directly on the object (“By Object”)

3. Symbol Tables (Definitions)

Symbol tables contain named definitions used by objects.

Major symbol tables include:

Symbol Table	Contains
Layer Table	Layer definitions
Linetype Table	Linetype definitions
Text Style Table	Font and text settings
Dimension Style Table	Dimension formatting rules
Block Table	Block definitions
UCS Table	User coordinate systems
View Table	Saved views
Regapp Table	Registered applications

Example:

A LINE does not store the full definition of its layer. It stores a reference:

4. Layer Database

A layer is a database object containing properties and state information.

A layer definition includes:

Layer name
On/Off state
Frozen/Thawed state
Locked/Unlocked state
Colour
Linetype
Lineweight
Transparency
Plot/No Plot status
Viewport-specific overrides
Description (newer versions)

Important:

A layer does not contain the objects drawn on it.

Instead:

5. Block Definitions and References

Blocks have two database components:

Block Definition

The stored description of the block.

Contains:

Geometry
Attributes
Base point
Internal objects

Block Reference (INSERT)

The placed instance.

Contains:

Location
Rotation
Scale
Layer
Visibility state

Example:

6. Layout and Paper Space Information

The database stores:

Model Space

The actual design environment.

Paper Space Layouts

Contains:

Viewports
Title blocks
Notes
Sheet settings
Plot configurations

Each layout has:

Page size
Plotter
Scale
View settings

7. Named Object Dictionaries

These are database containers holding specialised information.

Examples:

Named views
Layer states
Visual styles
Materials
Multileader styles
Plot styles
Sheet set information
Custom application data

8. Dimension and Annotation Systems

Stored separately from geometry:

Includes:

Dimension styles
Annotation scales
Text styles
Multileader styles
Tolerance settings

A dimension object contains:

Measurement references
Formatting rules
Style reference

9. Display and Visibility Controls

Visibility is managed by several interacting database elements:

Control	Stored where
Object exists	Object database
Object layer	Object property
Layer On/Off	Layer table
Layer Frozen	Layer table
Viewport layer freeze	Viewport override
Object visibility	Object property
Transparency	Object or layer
Clipping	Viewport/object
Xref visibility	External reference database

10. External References (Xrefs)

The DWG database can contain references to other drawings.

Stored information:

File path
Reference name
Position
Scale
Rotation
Layer overrides
Reload status

The external drawing remains a separate database.

11. System Variables and Drawing Settings

These are database settings controlling behaviour.

Examples:

Units
Limits
Snap settings
Grid settings
UCS settings
Annotation settings
Selection behaviour
Display options
Plot settings

Examples of system variables:

12. Relationships Between Database Elements

The key concept is that AutoCAD is object-oriented.

Objects do not exist independently.

A simplified relationship:

The Most Important Concept

The traditional explanation of layers as “transparent sheets” hides the real structure.

A more accurate description is:

An AutoCAD drawing is a database of intelligent objects. Layers are classification and control records that objects reference. Objects carry geometry and can inherit or override display properties from those records.

This database view is the foundation for more advanced layer management systems such as your composite-coincident-overlay modelling approach, because it treats layers as data classification and control mechanisms, not as drawing surfaces.