>I am looking at a Word document (created by someone else) and see that both the Header and the Footer has a horizontal line. What is strange is that I cannot click/select this line to see the properties. Also, the line seem to go exactly from left to right margin.
>How would you think this line was created?

Creating this line is very easy. Select the header (or footer), Then type a series of ------ and press Enter. That's all.