ASCII Table: What Is It?

Every key you press on a keyboard, inside the computer, isn’t a letter — it’s a number. The ASCII table is what decides which number stands for which character, and that’s why a text file, opened in any program, always shows back the same letters.

What Is the ASCII Table, and Where Does the Name Come From

ASCII stands for American Standard Code for Information Interchange. According to RFC 20, the technical document edited by Vint Cerf that reproduces the official table, the standard exists for the general interchange of information among data processing systems, communication systems, and the equipment connected to both.

Before it existed, computer manufacturers had no shared way to represent characters. The IEEE’s Engineering and Technology History Wiki (ETHW) notes that before 1963, they used more than sixty different ways of doing it, to the point that machines couldn’t communicate with one another. In May 1961, IBM engineer Bob Bemer wrote to the American National Standards Institute (ANSI), proposing a single code for computer-to-computer communication. ANSI formed the X3.4 committee, led by John Auwaerter of the Teletype Corporation, and in 1963 published the standard we now call ASCII.

Binary code, the system of 0s and 1s a computer uses to represent any piece of information, sits underneath every character encoding: ASCII is simply the agreement on which sequence of 0s and 1s stands for which letter.

How the Table Works: A Number for Every Character

ASCII uses 7 bits per character, according to RFC 20, which labels the bit positions from b7, the high-order bit, down to b1, the low-order one. Seven bits allow for 128 different combinations, and 128 is exactly the number of characters the ANSI committee packed into the table: letters, digits, punctuation, and control codes.

Practical example: RFC 20 shows the math for the letter K, found at column 4, row 11 of the table: that position corresponds to decimal value 75, or the bit sequence 1 0 0 1 0 1 1. The letter O, at column 4, row 15, works out to 79 through the same method (column times 16, plus row). The word “OK” becomes, for the computer, the number sequence 79, 75 — a text is nothing more than a string of values like these, one after another.

Each block of values in the table holds one type of character.

Range	Decimal values	Examples
Control characters	0-31	NUL, BEL, LF, CR
Space and punctuation	32-47	space, !, #, /
Digits	48-57	0-9
Uppercase letters	65-90	A-Z
Lowercase letters	97-122	a-z
DEL	127	delete

The control characters, in the table’s earliest rows, give instructions to the device receiving the text. RFC 20 spells out several of them, including BEL, meant to call for human attention and capable of triggering alarm devices, and HT, a formatting character that moves the print position to the next tab stop along the line — applicable to display devices too, and to the skip function on punched cards.

The Extra Bit and Extended ASCII

A byte, the unit a computer handles data in most comfortably, is made of 8 bits — one more than the 7 ASCII uses. RFC 20 suggests exactly this: packing standard 7-bit ASCII inside an 8-bit byte, leaving the high-order bit at zero.

That eighth bit, once switched on, opens up more combinations: two to the seventh power is 128, two to the eighth is 256, double the original set. Those extra 128 positions, past 127, are what’s commonly called Extended ASCII.

Why Non-ASCII Characters Fall Outside Standard ASCII

The members of the ANSI committee, ETHW explains, were working within the limits of 7-bit hardware and could only create 128 characters. Realizing that wasn’t enough for a truly global system, Bob Bemer himself developed a method that let computers switch from one alphabet to another using escape sequences; more than 150 different “extra-ASCII” alphabets have been built since.

As a result, standard ASCII can’t represent accented letters, ideograms, or alphabets beyond the basic Latin one used in English. Even at the international level the standard stayed a 7-bit code: the ITU’s (International Telecommunication Union) Recommendation T.50, which defines the International Reference Alphabet, corresponds to the ISO 646 standard, the international version of ASCII, still on 7 bits.

The Unicode Consortium notes that before Unicode, hundreds of different encodings existed for assigning numbers to characters, none able to cover every language in the world on its own, and that two encodings could even use the same number for different characters, creating conflicts. Unicode was built to solve exactly that: it assigns a unique number to every character, regardless of platform, program, or language, keeping ASCII’s values as its first 128 codes while going far beyond them.

Where You’ll Still Find It Today

ETHW notes that ASCII became ubiquitous with the spread of the internet, since it formed the basis for characters in email messages and HTML documents: the history of the internet runs through this same agreement on how to represent letters inside a data packet.

Adoption took years to catch up, though: when IBM launched the System/360 in 1964, development lead Frederick Brooks decided its printers and punch cards weren’t ready for ASCII, and the company stuck with the older EBCDIC standard. Only the Univac 1050 and Teletype’s typewriters adopted ASCII during the 1960s; it took the first IBM personal computer, in 1981, to make it the genuinely dominant standard. Anyone who wants the full arc, from vacuum-tube calculators to personal computers, can find it in the history of computers.

ASCII still sits under the surface of nearly every programming language: many, as the Recap on PHP shows, ship ready-made functions for converting a character to its numeric value and back — the same operation RFC 20 walks through for the letter K.

Windows moved away from ASCII toward Unicode with its NT operating system, released toward the end of the 1990s, according to ETHW. The 7-bit code from 1963 still serves as the foundation Unicode builds on for its first 128 values, the starting point for every character table that came after.