Quoting

Words break at white space and semi-colons, except:

• Double quotes prevent word breaks:
  

      set a "x is $x; y is $y"

• Curly braces quote everything, including newlines:
  

      set a_script {
          expr $b * $c
          puts "we're saving this script for later" 
      }

• Backslashes quote special characters:
  

      set a word\ with\ internal\ spaces

Substitutions don't change word structure:

set a "two words"

set b $a