\cal - turns on calligraphic mode; only affects uppercase letters and digits.
{ \cal ... }
\cal command turns on calligraphic mode; only affects uppercase letters and digits.
\cal ABCDEFGHIJKLMNOPQRSTUVWXYZ
\cal 0123456789
\cal abcdefghijklmnopqrstuvwxyz
abcdefghijklmnopqrstuvwxyz
{\cal AB}AB
\cal AB \rm AB
\cal{AB}CD