How to use the ifsym package?

I have added \usepackage{ifsym} in the front, and LaTeX cannot pass the command \Letter. What should I do to it?

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Welcome to TeX.sx! Please add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}. –  Marco Daniel Mar 1 '13 at 12:40
Try \usepackage[misc]{ifsym}. –  cgnieder Mar 1 '13 at 12:44

By default the package only sets up the command \textifsymbol and a couple of font selection commonds, such as \textifsym, which will produce LCD type digits and certain chemistry related symbols. To get the Letter icon you can write \textifsymbol{0}. Of course that is not convenient. In order to use the more meaningful command names, you need to load the package with one of its options:

• clock - provides \showclock and different clock symbols
• weather - weather symbols \Sun, \Rain,...
• misc - includes the \Letter icon you are after
• alpine - map symbols for alpine regions e.g. \Hut, \Summit,...
• geometry - circles, squares, diamonds,...
• electronic - digital signals

So as cgnieder says \usepackage[misc]{ifsym} will provide the \Letter command.

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\usepackage[misc,clock,geometry]{ifsym}

in the [ ] include the sub-packages that you want... (You have the options above...:-) )