If you write a backslash before a carriage return, then the next line will be considered to be a continuation of the line you just ended with exactly one space in between. You can indent the continuation line and Tcl will still see just one white space. This is Tcl's solution to a common problem of how to enter long strings without messing up your indentation. This form of backslash substitution happens before anything else. So the above example would work just as well if curly brackets were used instead of quotes.
Substitution happens only once.
Square brackets obtained with variable substitution from a string value do not then generate command line substitution. And so on. The substitution involves the next four characters which must make a hexadecimal number representing the 16 bits of the unicode. Here is an example where a Danish word is placed in a string. Do not try this with a version of Tcl earlier than 8.1.