I was reading the book 'The Invisible Computer' by Donald A. Norman. In chapter
8 he discusses 'conceptual models' the working of the ruler in Microsoft Word.
It looks similar to the ruler used in OOo. The only difference is that Word has
a rectangle just below the 2 triangles which can be used to set the indention
(upper) and the margin. This rectangle can be moved to move both the indention
and the margin. OOo has this functionality when moving the lower triangle.

I have to admit I prefer the way Microsoft did it, having a special element for
moving both. Yet Donald notices some deficiencies of the visual elements used to
present this functionality.

Now I quote:
"The proper graphical design would show an explicit linkage between the
rectangle and both sliders. One way of doing this would be to make the rectangle
extend in length between the two controls: If I move a control, only that part
of the margin is affected, and the rectangle would grow or shrink accordingly.
If I moved the rectangle, it would move in position without changing its length,
thereby reinforcing the conceptual model of the operation. As it is, the
relationship is more difficult to discover and remember than necessary."

I really like what Norman suggested and that's why I would like to share it with
you.