Memo: how to type doubledash "--" and underline "_" in LaTex?

How to type double dash in LaTex?

Sometimes we need to type "--" in LaTex, but the parser of Tex will deal with it into one dash.

If you try "- -" a space in two dash, it is not the same with "--".

So you can try

• \texttt{ }: I think it is good but the format and size of character should be customized.
  
• -{}-: Excellent. Nature!

Reference from

http://www.latex-community.org/forum/viewtopic.php?f=44&t=4670&start=0

Underline is reserved character of LaTex

The reserved characters are

• $ : dollar sign
• & :
• %
• #
• _
• {
• }
• ~
• ^
• \
  

If you want to type them, just add a "\" before them.

http://www.agu.org/meetings/mtabsLTX.html