These are present in the San Francisco class bravo / Oakland class charlie and O'Hare class bravo / Midway class charlie.
T is used to signify that the top of Class C airspace that lies under Class B is the bottom surface of that airspace. This is important when the Class C segment lies under multiple layers of Class B, where no single top altitude applies.
You can start many "what does this thing on an FAA chart mean" questions with the FAA's Aeronautical Chart User's Guide. It doesn't go into a deep explanation, but does show it as an example in this case.
The symbol can be found on page 17 of the 2018 guide.