Do TTY, TDD, and TT mean the same thing?

DO-IT Factsheet #64
http://www.washington.edu/doit//articles?64

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 AccessIT article What access challenges do people with disabilities face when using a telephone? [1] and consult Telecommunications Relay Services [2] and Telecommunications Products [3].

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