I've noticed that some outlets have a T-shaped neutral slot. What does this do?

It identifies the outlet as a 20-amp grounded receptacle and it should be used only on 20 amp circuits.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.