How does one insert a "" (backslash) into the text of a LaTeX document? And how does one insert a "~" (tilde)? (If you insert ~, it will give you a tilde as an accent over the following letter.) I
How does one insert a backslash or a tilde (~) into LaTeX?
Possible Duplicate: How to look up a symbol? Escape character in LaTeX In the itemize environment, how do I use the ampersand symbol & without LaTeX trying to use it as a command?
How do I use '&' literally in LaTeX? - LaTeX Stack Exchange
Sometimes the symbol := is used to denote a definition. For example, X:=Y+Z means that X is defined to be Y+Z. When using LaTeX, can I just use $:=$, or do I need to do something special?
How to type these special letters from European languages in latex? ä, é, and L'?
The LaTeX command is \symbol. You can see that LaTeX default underscore does not use char 95 when encoding is OT1, because it occasionally fails depends on the font (i.e., the encoding does not guarantee that the character at position 5F (hex) is an underscore, that character in the specific font cmtt10 "happens" to be an underscore)
LaTeX still supports it for one reason or another, but the "proper" one to use, as defined in the specifications, is [ ... ]. All this just means that they are not promising that $$ will always work. So it is technically possible (though unlikely in the near future), that compatibility with $$ is removed from LaTeX and lots of your documents ...