It is interesting - like the BR for break, the horizontal line does not have a closing /HR. Therefore we use the same code we used for br with the / included with the hr.
If I don't want the line to go all the way across the page, I can do a width on my horizontal line. And if I want it to be wider I can change the thickness by using the SIZE clause. To make the size a solid line, I also need the noshade clause. Notice that the line is centered. However to be safe in a variety of browsers, it is probably better to use ALIGN=CENTER.
If I want the line to start at the LEFT, I need to align it to the left.
If I want the line to always go across 25% of the screen without worrying about the number of characters, I can use percent to set the width.