The US Army Corps or Engineers produces a nice collection of CAD Standards, which includes numerous symbols, linetypes and hatch patterns. Use the link below for more information.
Details/Download: https://cadbim.usace.army.mil/CAD
These system variables are not documented but are still very useful.
Command: _PKSER (Returns the AutoCAD serial number)
Command: _VERNUM (Returns details about the installed version)
See a more complete list at ManuSoft’s site.
By default, AutoCAD does not display images (only the border) during most pan/zoom operations . If you would like to display images in these cases, change the RTDISPLAY system variable to 0.
This little known command has a special purpose. One case is when you have just created a block definition and want the source objects back. Using an UNDO would undo the newly created block, the OOPS command brings the object back without undoing the block.
A second (more likely) case is when you simply want to undelete the last object(s) that were erased, without undoing.