Welcome to the world of HTML. This is the first paragraph. While short it is still a paragraph!
The horizontal rule separating these two paragraphs is the default one. It is definitely horizontal, but the default horizontal rule does not expand your horizons as much as the thicker rules do! :-)
Here is a thicker rule. The command is HR SIZE=10 as a tag.
Here is a darker rule. There are many options in HTML. The command is HR NOSHADE inside of a tag.
And here is a cascade of horizontal rulers in successively smaller sizes and successively smaller widths.