A TTY device, or teletypewriter, is a specialized machine that allows individuals, particularly those who are deaf or hard of hearing, to communicate over telephone lines. It converts typed text into signals that can be transmitted and received through standard phone systems, enabling real-time text conversations. TTY devices typically feature a keyboard for typing messages and a display screen for reading incoming text. They can connect directly to a phone line or be used with a relay service, where an operator assists in converting text to voice and vice versa, ensuring effective communication between TTY users and those using traditional phones.