Thickness is a property of certain objects that gives them a 3D appearance.
The 3D thickness of an object is the distance that object is extended, or thickened, above or below its location in space. Positive thickness extrudes upward in the positive Z direction; negative thickness extrudes downward (negative Z). Zero (0) thickness means that there is no 3D thickening of the object. The Z direction is determined by the orientation of the UCS at the time the object was created. Objects with a non-zero thickness can be shaded and can hide other objects behind them.
The thickness property changes the appearance of the following types of objects:
Modifying the thickness property of other types of objects does not affect their appearance.
You can set the default thickness property for new objects you create by setting the THICKNESS system variable. Change the thickness property of an existing object using the Properties palette. The 3D thickening is applied uniformly on an object; a single object cannot have different thicknesses for its various points.
You may need to change the 3D viewpoint to see the effect of thickness on an object.