Lineweight determines the thickness of a line and you can configure it using the lineweight menu of the Properties panel, as shown in the following figure:

Once again, to explain this, I will take the example drawing that we have used so far in this section. Here, I will make the outer boundary of the drawing thicker than the internal line and circle so that the end result looks like drawing B:

The default value for lineweight is ByLayer, which applies a theoretical thickness of 0.00 units, which obviously is not possible in the real world but in AutoCAD lines can theoretically have a thickness value of zero. However, you can change this and add any thickness value from the lineweight list. 

To explain this, I will add a lineweight of 0.5 units to the outer boundary of the preceding drawing using the following steps:

1. To add the lineweight, select the outer boundary from the drawing area.
2. Click the lineweight menu to expand it and from the list, select the lineweight that you want to apply. In this case, select a lineweight of 0.50 units. The units can be millimeters or inches, depending on the unit you have specified in the drawing template.
3. Once the lineweight is applied, press the Esc key to remove the objects from your selection and the boundary will now look bold with a thickness of 0.5 units. This thickness is relative thickness and if you zoom in or out, the thickness will always look the same.

If in this case too, after applying the lineweight, you don't see any change in the drawing, then activate the Lineweight display status bar option as shown in the following figure. If you don't even have the Lineweight display status bar option on the status bar, then click on the three-lines icon at the end of the status bar and select the Lineweight option:

Once the Lineweight option is visible on the status bar, make it active and you will be able to see the lineweight applied to objects in the drawing area.