This is very important, so it's worth taking a moment to give an example. If you have a window that you wish to drawing 5-pixel-wide solid (not dashed) lines in, and you want the ends of the line segments to be rounded, you might do:
graphics_attributes(Window, [line_width(5), cap_style(round)])
That's all there is to it. You don't need to specify the
line_style(solid)
attribute, because this is the default. You may
count on the defaults; they are often what you want anyway. It is
perfectly possible to create a window and start drawing into it without
setting any graphics attributes, because the defaults may be good
enough.