Do TTY, TDD, and TT mean the same thing?
Yes. The TTY (TeleTYpe), TDD (Telecommunications Device for the Deaf), and TT (Text Telephone) acronyms are used interchangeably to refer to any type of text-based telecommunications equipment used by a person who does not have enough functional hearing to understand speech, even with amplification. The person sending a message types it on a small keyboard, and the person receiving the message reads the text on the display. In order for a person to use a text telephone, the individual at the other end of the conversation must also have one, or they must use a relay service. A relay service provides an operator who reads the text on a TTY and speaks the message into a telephone and/or listens on a telephone and types the message on a TTY.
For more information regarding these devices and how they work, consult the article What access challenges do people with disabilities face when using a telephone? and consult Telecommunications Relay Services and Telecommunications Act Section 255 Accessibility Guidelines.