At a place where a command is expected, the char # serves as [comment]. That can be the beginning of a line or immediately after a semicolon (;). A line or bracketed [[expression]] beginning with # is treated as ''nothing''.

The # differs from the construct if 0 ... such that its following words are not processed as arguments of a true procedure but instead are ''really ignored''.

If you write this:

 if 0 [puts a]

then the output is a, followed by the prompt:

 a
 %

[RJ] Not in my universe:

    > if 0 {puts a}
    > 

[wdb] The difference is: you used curly { braces } -- I used [[ brackets ]] (which is not propagated as nice programming style).

If you write this:

 # [puts a]

then nothing happens:

 %

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[AMG]: [http://tip.tcl.tk/148%|%TIP 148] is relevant.  If the first word of a list starts with '''#''', it is now brace-quoted to ensure that if the list is passed to [[[eval]]], it is treated as a command rather than a comment.

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''See also:'' [if 0 {]

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