By default, all objects except circles, full ellipses, closed polylines, and splines are trimmed when filleted. You can use the Trim option to specify that filleted objects remain untrimmed.

To fillet without trimming

From the Modify menu, choose Fillet.
Enter t (Trim).
Enter n (No Trim).
Select the first line and then the second line.

The filleted object is untrimmed.
System variables […]
Source: CAD Blog.Net – Trimming Filleted Objects in AutoCAD
Go to Source: CAD Blog.Net