How to open multiple drawings in AutoCad

Issue:

Attempts to open multiple drawings within the same AutoCAD session fail, and the user is prompted to save the current drawing, prior to opening or creating any other drawing.

Causes:

This behavior occurs because the SDI system variable is not set to its default value, which is 0. The SDI (Single Document Interface) system variable is legacy, and is maintained within the program for backwards compatibility. SDI being set to 1 (or 3 in some cases) will enable the single-document mode, in which only one drawing is allowed to be open at a time.

This variable has largely been made obsolete, though, and no longer appears in AutoCAD's Options dialog nor the Help documentation. 

Solution:

To disable Single Document Interface:

1. At the command prompt in AutoCAD, enter SDI.
2. Enter 0 as the value.

The default value for SDI is 0. Some applications that were designed for earlier releases of the program may set the SDI system variable to a non-zero value for compatibility reasons.

SDI Settings

This list shows valid settings for SDI and explains what each setting does.

0	Multiple drawing interface enabled
1	Multiple drawing interface disabled
2	Multiple drawing interface disabled by an application that does not support multiple drawings
3	Multiple drawing interface disabled by user and by an application that does not support multiple drawings

Note: AutoCAD LT software only allows 0 or 1 for SDI because it cannot be used with add-on software. For AutoCAD products which are native to the Apple Macintosh (Mac) operating system (OSX), the SDI variable is set to 0, and is read-only.