A common description of AutoCAD layers is that they are like transparent sheets on which objects are drawn. While this analogy may help beginners, it creates a major misunderstanding.
A CAD layer is not a sheet, surface, or container holding objects.
All AutoCAD objects are created with a set of properties assigned by the current environment. One of those properties is its Layer Name controlled by the system variable CLAYER. Newly created objects are assigned that layer name automatically.
A layer is a named database definition containing default visual properties and drawing controls. Objects do not exist “inside” a layer; instead, each object is assigned a LAYERNAME property that references a layer definition.
Each object has three main groups of properties:
- Layer Property
The name of the layer assigned to the object.
- Geometric Properties
The actual shape and position of the object, such as points, lengths, angles, radii, and coordinates. - Visual Properties
Properties controlling appearance, including:
- Colour
- Linetype
- Lineweight
- Transparency
- Plot style
- Plot/No Plot status
Visual properties may be:
BYLAYER — inherited from the assigned layer definition.
BYOBJECT — individually assigned directly to the object.
Most professional drawings use BYLAYER because it allows many objects to be controlled by changing one layer definition rather than editing objects individually.
For example, if all wall objects are assigned to layer WALLS and their colour is BYLAYER, changing the WALLS layer colour automatically changes the appearance of all walls.
The LAYERNAME remains the object’s reference to that layer regardless of whether its visual properties are inherited from the layer or assigned directly.
Useful commands:
- CHPROP — change object properties
- MATCHPROP — copy properties between objects
- LAYMATCH — match objects to another layer
- LMAN — manage layer states
The power of layers is not that they contain objects. Their power is that they provide a controlled method of applying consistent property definitions to groups of objects.