By Dennis Collin
Working with simple elements
Many AutoCAD users are a little uncertain about what properties to set to items in terms of colour, linetype and lineweight, both when drawing normal elements and creating symbol libraries.
Every object has general properties including layer, colour, linetype, linetype scale, lineweight, transparency and plot style. When colour, linetype, lineweight, transparency and potential plot style are set to Bylayer, the objects behaviour will be defined by the Layer the element is currently set to.
I.e. Draw line set to Bylayer on a Layer that is set to a red colour and dashed linetype will appear as a red dashed line. If that line is moved to a different layer the objects appearance will changed accordingly.
When an object is set to a Byentity setting, then the object will override any equivalent layer display controls.
I.e. A line set to red and dashed linetype will always look red and dashed even if placed on a Layer that has a blue colour and solid linestyle applied.
Working with Blocks
Blocks are a little more involved, but they follow the same principle as basic entities in that, set to Byentity overrides the current layer. Bylayer follows the current Layer setting. ByBlock is like Bylayer where it follows the layer setting unless otherwise specified.
This is illustrated with a simple door block example below with a colour explicitly set to Red, Bylayer and Byblock with descriptions and images showing the effect of attempted changes. Similar controls can be achieved using settings for Linetype, Lineweight and transparency. Byblock settings are also available for more complex elements such as Dimensions and Multi-leaders.
In summary, what process should be followed? Well of course as with anything it depends upon the circumstance. However, as a rule I personally advocate the strict following of Bylayer settings as Layers should be the primary sorting method of elements within an AutoCAD drawing. Layers can then dictate the display and editing properties of elements. If some flexibility is required for blocks without creating additional layers, then Byblock settings can be used to change how a block looks without placing it on a different layer. However, conventional wisdom dictates if an element needs to look different then it should be on a different layer, therefore the Bylayer principle is the one that should be followed by most users.