You come across it everyday but you hardly spent any thought about it: the telephone’s 10 button keypad. It appears natural in its 3×3 arrangement, the numbers ascending from left to right and from top to bottom. The same layout is used on e.g. smart phones, ATMs, cell phones, pin entry devices and card payment terminals. However, the number layout is different on a calculator or numeric part of the keyboard (3×3 as well, but ascending numbers from left to right bottom to top).

While it feels easy to use, the 3×3 keypad number layout resulted from extensive user research conducted by Bell-Systems in the 1950ties. More exactly the research was conducted by John E. Karlin, leader of the Human Engineering department at Bell Lab. The department was the first of its kind at an American company.

In the early days of the telephone the keys were arranged in 2 vertical lines: ß-4; 5-9. A phone number for connection was dialled twice for long-distance calls in the connection process – first from the caller and second from the telephone exchange. Studies were conducted to evaluate the speed of number dialing, number of errors, and count of correct connected long-distance calls. The results of those studies suggested the 2-line key layout to be comparable more error-prone. 13% of long-distance calls were connected wrong.

A study specifically evaluated at which position users expected numbers to be, and in combinations of numbers and letters, at which position letters would be expected. The study compared speed, accuracy, and preference over 16 pre-selected layouts. Pre-selection criteria were prior knowledge, user expectations, and engineering requirements. Today’s number arrangement on the calculator was amongst the compared designs.

Each participant tried all designs, asked to key as quickly and accurately as possible (similar as being in office using the telephone). After the observational part of the study, the participants completed the layout sketches with their preferred order of the numbers in a questionnaire. After the study five favourable layouts emerged:

1. 3×3 plus 1 grid
2. Two horizontal lines
3. Two vertical lines
4. Circular layout from old dial phones
5. Speedometer circle

In case you wondered, the 3×3 number layout of the calculator was slightly slower compared to that of the telephone. So it did not make it into the top 5. Further, calculators were not everyday objects in the 1960ies as they are now and even the few being used they varied in layout. So different number layouts of telephone and calculator were not considered to be confusing.

The two two line layouts were excluded because the participants did not enjoy to use them. Because the 3×3 grid offered engineering advantages it was selected in favour to the two circular layouts.

Sources / Further reading:

1. Summary of the work, 1960, Bell Systems Technical Journal, “Human Factors Engineering Studies of the Design and Use of Pushbutton Telephone Sets“: http://datagenetics.com/blog/august32015/article.pdf

2. https://www.theatlantic.com/technology/archive/2013/08/the-17-designs-that-bell-almost-used-for-the-layout-of-telephone-buttons/279237/

3. https://historyofinformation.com/detail.php?entryid=1031

4. https://www.nytimes.com/2013/02/09/business/john-e-karlin-who-led-the-way-to-all-digit-dialing-dies-at-94.html

5. https://en.wikipedia.org/wiki/Telephone_keypad