Horizontal lines?

[Cal]

How do I draw horizontal lines in a post? There was an icon to click at the top of the old forum message box.

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Do I just do a series of underscores (as above)?

Edit: Which if I do (above was the full extent of the message box) it will either be too short (as above) or too long I guess.
Edit 2: When I pressed "preview" it showed up just the perfect length.

 

[Cal]

Advice from @Toothpick Jason:
"The horizontal line is [ hr] (with out the space in the brackets)"
Here goes:

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Edit: Fabulous Jason, thank you.

 

[Toothpick]

Yahtzee!