ASCII 31 — US

Unit Separator (US) is the lowest-level delimiter in ASCII's four-tier information separator hierarchy, marking boundaries between individual data fields within a record. US corresponds to a column separator in tabular data — performing the role that commas serve in CSV or tabs in TSV files. Using US (ASCII 31) as a field separator has the practical advantage of being virtually impossible to confuse with actual data content, unlike commas, tabs, or pipes that frequently appear in text and require complex escaping rules.

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, Unit Separator is assigned code point 31 in decimal (0x1F hexadecimal, 037 octal, 00011111 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 Unit Separator works reliably across all operating systems, programming languages, and internet protocols.