Editing Multiline Intersections in AutoCAD
If you have two multilines in a drawing, you can control the way they intersect. Multilines can intersect in a cross or a T shape, and the crosses or T shapes can be closed, open, or merged.
To create a closed cross intersection
From the Modify menu, choose Multiline.
In the Multiline Edit Tools dialog box, click Closed [...]
Source: CAD Blog.Net – Editing Multiline Intersections in AutoCAD
Go to Source: CAD Blog.Net