To alter the Line Cap Type,click on one of the choices (Butt, Rounded, or Square) under the line cap heading. The selection will appear in dark grey as a visual indicator of the currently active line cap type.
For example, the image below shows what a Line would look like with a Butt, Rounded, and Square Line Cap.
A line cap change will be seen instantaneously when a stroke figure is selected. If no figures have been selected, the line cap change will apply to all subsequently created stroke figures.