Layers in QCAD define how your entities look like (color, line type and lineweight).
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Layers are a fundamental concept in CAD. Each line, arc, etc. you draw resides on a layer. Layers are part of your drawing and stored with your drawing, so every drawing has its own set of layers. All entities with the same type of real world function are typically residing on the same layer. Since the layer defines the look of all entities on it, only entities with the exact same look are usually put on the same layer.
For a mechanical drawing, you might create a layer "hidden" for hidden edges (thin, black, dashed) or "center" for center lines (thin, red, dash-dotted).
For an architectural drawing, you might create a layer for interior walls, one for exterior walls, one for electrical installations, etc.
Step 1: Showing the Layer List
When working with layers, make sure that the layer list is visible. If that is not the case choose menu View > Layer List.
The layer list shows all layers of the current drawing. It also shows which layer is currently active - that means which layer you are drawing on. Layers can also be added, deleted, switched on / off, etc. all from the layer list.
Step 2: Creating a Layer
Use the Layer > Add Layer tool to create a new layer. Call the layer "Layer 1" and give it a color (red), lineweight (0.13mm) and linetype (dash dot):
In the same way, add another layer "Layer 2" with different attributes (e.g. "green", 0.2mm, Continuous).
The default color and lineweight for new layers and for layer "0" in new drawings can be configured under Edit > Application Preferences > Layer > Add Layer.
Step 3: Using Layers
The layer list always shows you which layer is currently active: the one that is highlighted in the list. To activate a different layer, simply click on it in the layer list.
Click on layer "Layer 1" and draw some lines. The lines are drawn in red, with the attributes previously chosen for that layer.
Click on layer "Layer 2" and draw some more lines. These lines are drawn in green:
Step 4: Changing Layers
Double-click "Layer 1" in the layer list. The layer dialog is shown again, this time to allow you to edit the layer. You can change the layer name or its attributes.
Change the layer colour to "Yellow" and click OK.
As you can see, the lines that were previously drawn while "Layer 1" was active are now yellow:
Layers allow you to manage the attributes of all related entities at once.
Step 5: Hiding Layers
Click the eye icon next to "Layer 1". It turns grey, meaning that this layer is now turned off. All lines that reside on "Layer 1" are now hidden. They are still part of your drawing and will also be saved. They are just not displayed anymore. This can be useful to hide certain information in a drawing (for example electrical installations) while working on other parts of the drawing (for example interior walls).