Help with Chapter seperators (lines underneath chapter headings)

Hi,

I can't figure out how to create bold lines beneath chapter headings.

I've spotted many kindlebooks that make use of these, but I have no idea how they do it!

I am able to use a simple underscore and run it along the page, but even when this is (h1 size) it doesn't look that bold.

I've spotted books that use fancy lines too. If anyone can help me with this, it would be much appreciated. Thanks! :help:

Sarah.