Curly braces { } provide another way of grouping information into words. They are different from quotes in that no substitutions are performed on the text between the curly braces:
set z {$x + $y is [expr $x +
$y]}
This above example sets variable
z to
the value "$x + $y is
[expr $x + $y]".
Curly braces also group words into a single argument. In this case, however, elements within the braces are not interpreted. Braces can speed up the execution of your code. If a command statement does not update as expected, try switching to Quotes.