When the ASCII code is used to communicate between computer terminals, each terminal can send an enquiry character to request the condition of the other. The receiver of this character can respond with ACK (00001102 or 616) to indicate that it is operating normally, or NAK (00101012 or 1516) to indicate an error condition. Unicode provides visible symbols for these characters, U+2406 (␆) and U+2415 (␕).