There may be any number of variable substitutions in a single word. Variable substitution is not performed on words enclosed in braces.
Note that variables may contain character sequences other than those listed above, but in that case other mechanisms must be used to access them (e.g., via the set command's single-argument form).
Backslash substitution is not performed on words enclosed in braces, except for backslash-newline as described above.
Substitutions take place from left to right, and each substitution is evaluated completely before attempting to evaluate the next. Thus, a sequence like
set y [set x 0][incr x][incr x]
will always set the variable y to the value, 012.