Create panel > Geometry button > Extended Primitives > Object Type rollout > L-Ext button
Create menu > Extended Primitives > L-Extrustion
Tab panels > Objects tab > L-Extrusion Object
Use L-Ext to create an extruded L-shaped object.

Example of L-Ext
To create an L-Ext object:
From the Create menu, choose Extended Primitives > L-Extrusion.
Drag the mouse to define the base. (Press CTRL to constrain the base to a square.)
Release the mouse and move it vertically to define the height of the L-Ext.
Click, and then move the mouse vertically to define the thickness or width of the walls of the L-Ext.
Click to finish the L-Ext.
Corners—Draws the object from corner to corner. You can change the center location by moving the mouse.
Center—Draws the object from the center out.

Side/Front Length—Specify the lengths of each "leg" of the L.
Side/Front Width—Specify the widths of each "leg" of the L.
Height—Specifies the height of the object.
Side/Front Segs—Specify the number of segments for a specific "leg" of the object.
Width/Height Segs—Specify the number of segments for the overall width and height.
Note: The object's dimensions (Back, Side, Front) are labeled as though it were created in the Top or Perspective viewports, and seen from the front in world space.
Generate Mapping Coords—Sets up the required coordinates for applying mapped materials to the object. Default=off.
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