The AutoCAD linetype syntax is pretty simple. Referring back to Part 1, recall the linetype definition that I copied, edited, and named MY-DASHED.
Looking at the definition, the first item is an asterisk, followed by the name of the linetype, then a short description, and finally an ASCII representation of the linetype. This last part is totally up to you, just get it as close as you can. Look at some of the other linetypes as a reference. Read more about AutoCAD Linetypes - Part 2
So where do AutoCAD linetypes come from? When you start up AutoCAD and start drawing some geometry and decide to change the linetype to something other than Continuous, where did they come from? How did they get there? How can you change them or add more?
To start with, there are two types of linetypes, simple and complex; both are stored in the DWG file. Simple linetypes are composed of line segments and spaces (or gaps) only. Examples include the HIDDEN, DASHDOT, and PHANTOM linetypes, as shown below. Simple linetypes are fully contained in the DWG file. If you send a DWG file to someone else, you do not have to send any external files along also in order for the simple linetypes to display correctly.
Read more about AutoCAD Linetypes - Part 1
In AutoCAD, you can assign a non-continuous linetype to a 3D polyline, but it will not display or plot.
If you need 3D polylines, and you need a non-continuous linetype, for example to represent the flowline of a ditch - there are a couple of workarounds. Both assume that you have assigned the correct linetype to your 3D polylines.
If you have ever tried making a custom linetype with inch symbol (") in the linetype definition, you know AutoCAD will complain, because it interprets this as an extra quote mark in the linetype string. Instead, use %%34 to represent the inch mark ("). See the following example: