ASCII 15 — SI

Shift In (SI) returns the output device to its default character set after a Shift Out (SO) command switched to an alternate set. This pair of control characters enabled early terminals and printers to display extended symbol sets without wider character codes. The shift mechanism influenced the design of stateful encoding schemes and codepage switching systems like EBCDIC used in IBM mainframes. In modern VT100-compatible terminals, SI deactivates the G1 character set and restores the default G0 set for normal text display.

Control characters were defined in the original 1963 ASCII standard to manage telecommunications equipment and terminal devices. Unlike printable characters representing visible symbols, control codes perform actions: initiating transmissions, acknowledging received data, triggering device alerts, and structuring information hierarchically. Of ASCII's 128 code points, 33 are designated as control characters (codes 0–31 plus 127), reflecting the standard's deep roots in telegraphy and serial communication systems. While most control codes have fallen out of daily use, several remain essential to modern computing workflows.

In the ASCII encoding table, Shift In is assigned code point 15 in decimal (0x0F hexadecimal, 017 octal, 00001111 binary). The 7-bit ASCII standard, first published in 1963 by the American Standards Association, defines exactly 128 characters that remain the foundation of text encoding systems worldwide. UTF-8, the dominant encoding on the modern web, is fully backward compatible with ASCII — every ASCII character is encoded as the identical single byte in UTF-8, guaranteeing that Shift In works reliably across all operating systems, programming languages, and internet protocols.