If a hash character ("#") appears at a point where Tcl is expecting the first character of the first word of a command, then the hash character and the characters that follow it, up through the next newline, are treated as a comment and ignored. The comment character only has significance when it appears at the beginning of a command. (From: [TclHelp]).

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Tcl students frequently ask "[Why can I not place unmatched braces in Tcl comments]?"

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Tcl comments are almost like null-operation commands. You could write one yourself like this:
 proc -- args {}
 -- This is a call to the new proc that does nothing, like a comment
 -- but this here still does an [exit]!! So it's not like
 # [exit] [exit] [exit] ... and still won't die ;-)

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Note the "if 0 ..." [idiom]:
 if 0 {
 Any Tcl code to be commented out (with matching braces, of course!)
 or any other kind of text, will be ignored - and even [exit] won't fire
 because it's in braces, so left unevaluated!
}
and a fancy sugar for that: "#" is special only if first character of a command name. Nobody hinders you to write a 
 proc {#} args {}
where the braces are only visual markup - the parser strips them off, the proc's name will be just "#". But to call it, you have to escape the "#" sign - with a backslash, or with my favorite sugar:
 {#} {
   This is a comment in braces
 }
where the name of the command is pretty self-documenting: "comment in braces"... Much more intuitive than e.g. [Ruby]'s "=begin ... =end", and available with almost zero implementation! ([RS])

In the [Tcl chatroom], [MS] pointed out: "the bytecode compiler recognizes "proc whatever args {}" and compiles no invocation for that; the cost is (almost) nothing. For other empty procs, an invocation will be compiled, so that you have a call/return overhead", and that "if 0 {...}" may raise errors on some contents (up to tcl8.3). Any word corresponding to a bcc'ed command at the start of a line is risky ...:
 % if 0 {set a b c}
 % if 0 {if you want to write this, you can't}
 % if 0 {while you read this, tcl errs out}

Therefore,
 {#} {set any number of words; if you so wish} 
is more robust than "if 0 {...}", as long as you DO NOT forget the braces, and before 8.4. But 
 {#} do not [[exit]]
*will* exit ...

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[AB] Forgive me for an easy question, but is there a way to comment out a whole block of code in TCL? For instance, I know in C++ you can do so by typing "/*", and then everything that follows is commented out until the compiler comes to "*/". Thanks! 

Use
  if {0} {
     ... code to comment ...
  }

[MG] Tcl doesn't have -real- block-commenting like that, though. The ''if {0} { ... }'' method won't work if you have mismatched braces inside, or anything like that - the first mismatched ''}'' will end the ''if'', and the code after it will start running.

(noname) 18-Aug-05 Try it! Use
  # First, define an block comment procedure, as:
  proc /* {args} {}
  # Then,you can use it as:
  /* { ...
  .. any comments,(with carefull { } pairs).
  } and more args in this line will be ignored.

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