Why are there no Letters on the "1" Key on Mobile Phones?

Why are there no letters on the “1” key on mobile phones, while the “2” to “9” keys have several? It might be a small detail, but once you notice it, it’s hard to ignore. Can you see it now?

The fact that there is no letter on the “1” key is not a design flaw, nor is it a phenomenon that emerged with the advent of mobile phones. The answer lies much further back, in the history of phone numbers and the technical realities of the earliest telephone networks, and this is what we will explore in this article.

Long before mobile phones, texting, or even keypads, phone calls were handled by human operators, where you would pick up the receiver and ask to be connected. As telephone usage skyrocketed in the early 20th century, this system quickly became quite impractical. Thus, phone companies began automating their networks using rotary dials and mechanical switching equipment that could interpret sequences of numbers.

It was during this transition that telephone exchange names emerged — a distinctive and easy-to-remember name assigned to a central office. It identified the switching system to which a telephone was connected and facilitated the connection of phone calls between switching systems in different locations. The first letters of a telephone exchange name were used as the main components of the telephone number representation, so that each telephone number in the area was unique. These letters were associated with the dial numbers, which were visibly indicated on the dial plate.

Since exchange codes were not allowed to begin with 0 or 1, only digits 2-9 were used for letters, leaving 1 without any associated letters.

This moment marks a significant turning point in the history of phone numbers, as it combines human-friendly naming with machine-readable digits.

A standard telephone keypad has ten digits, but only eight of them are designed to represent letters. This choice is no coincidence. From the outset, engineers reserved the digits “0” and “1” for specific purposes:

• “0” was used to reach the operator and for specific network services; and
• “1” was used as a signaling digit, often indicating how a call should be routed.

In early switching systems, these digits were not just numbers, but also instructions. Assigning letters to them would have made the system more difficult for machines to interpret in a reliable manner.

For instance, if people had assigned letters to the “1” key, these letters could have appeared naturally in a word or an exchange name. It would have posed a serious problem as the number “1” was (and often still is) used for things such as:

• Long-distance or interurban call prefixes;
• Short service numbers (emergency, assistance, network access); or
• Internal signaling within the telephone network

Thus, allowing the “1” key to carry letters would have confused early switching equipment and disrupted automatic call routing. That is why, in the early decades of automated telephony (around the 1920s and 1930s), people decided not to assign a letter to the “1” key. This choice then became the norm, even on modern smartphones.

Recommendation E.161 is an ITU-T recommendation, which specifies the layout of numbers, letters, and symbols on a telephone keypad and dial. It also defines the recommended correspondence between the basic Latin alphabet and numbers (for example, the letters "DEF" on the number 3). This correspondence is used for various purposes:

• Entering letters by successive keystrokes and predictive text systems (intuitive entry for sending messages or SMS);
• Transforming telephone numbers into phonewords; and
• Using alphabetic characters (e.g., as mnemonics) for a PIN code.

The telephone keypad is specified in the standard 4 × 3 layout (4 rows, 3 columns) and with several variants, such as 6 × 2 and 2 × 5. Recommendation E.161 also specifies the dimensions and characteristics of the "star" and "square" keys. In practice, on new phones, the "square" key is almost invariably replaced by the ASCII character # (number 35). Want to know more about it? Check out our ASCII converter.

Overall, this evolution is a natural part of the broader history of mobile phones, showing how modern interfaces still rely on decisions made nearly a century ago.

Because only the “2” to “9” keys contain letters, people had to find a way to make writing text practical on a numeric keypad. This limitation first led to multi-tap input, where users pressed the exact key multiple times to find the desired letters. Then, to simplify this process, intuitive or predictive text input was developed.

Interestingly, predictive text did not originate with mobile phones. It was first patented in the United States in 1985 as a method to facilitate telephone communication for individuals who are deaf. At the time, its applications were limited, and it wasn’t until the advent of mobile phones and their text messaging service that predictive text found its primary use.

The most widely adopted version of this technology was created by Tegic Communications and marketed under the name T9, an abbreviation for “text on 9 keys”. T9 was integrated into the keypad phones of nearly all major manufacturers, including Nokia, Samsung, LG, Siemens, and Sony Ericsson. Its impact was so significant that the name “T9” eventually entered everyday language, becoming a generic term used to describe predictive text input as a whole (even on devices that didn’t technically use Tegic’s system).

At its core, T9 works by interpreting number sequences as possible letter combinations, using a built-in dictionary to guess the most likely word. This mechanism is explained in detail in our following article: “How Does Predictive Text Work?”.

After reading this article, we hope you now have a better understanding of:

• The history of phone numbers; and especially
• Why there are no letters on the "1" key on mobile phones.

As you have just seen, the answer to the question "why are there no letters on the key ‘1’ on mobile phones" is not accidental. It may be an insignificant detail on your keyboard, but it is a perfect example of how choices made decades ago continue to influence our current use of technology.