Double quotation marks (" ") group words as a single argument to a command. Dollar signs and square brackets are interpreted (i.e. updated or evaluated) inside double quotation marks. When in doubt, use Quotes.
Double-quotes allow you to specify words that contain spaces. For example:
set x 88
set y 11
set z "$x + $y is [expr $x + $y]"
After these three commands are evaluated,
z will be a text
+ 11 is 99.
Everything between the quotes is passed
set command as a single
word. Command and variable substitutions are performed on the text
between the quotes. The quotes themselves are not passed to the
command. If the quotes were not present, the
set z command would have
received 6 arguments, which would have caused an error.