ASCII 1 — SOH

Start of Heading (SOH) marks the beginning of a message header in telecommunications. In early serial communication, SOH signaled that the following bytes contained addressing or routing information before the actual message body. While largely obsolete in modern networking, SOH persists in industrial serial protocols like Modbus and in point-of-sale terminal communication. Generated by Ctrl+A on keyboards, this code point is now universally remapped to the Select All shortcut in graphical text editors and word processors.

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, Start of Heading is assigned code point 1 in decimal (0x01 hexadecimal, 001 octal, 00000001 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 Start of Heading works reliably across all operating systems, programming languages, and internet protocols.