12.3.3         Curly braces { }

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.