Short message service (SMS) is the system through which people can send brief text messages between cell phones. In the English alphabet, these messages are limited to 160 characters each, including spaces. This limitation results from the system on which SMS operates. Instead of using the primary system for telephone communication, SMS uses a channel designed to send small packets of information between cell phones and cell towers. The 160-character SMS limit is simply a consequence of the originally modest use to which this channel was put. The channel was chosen as the basis for SMS because the system could be implemented easily, with minimal need for new infrastructure.

Cell phones are supported by a network of towers responsible for transmitting calls. These towers constantly communicate with the cell phones they support using a frequency called the control channel. The control channel is used to relay information about the location of phones and towers; it is also used to orchestrate calls. Unlike the calls themselves, these processes require only small exchanges of information. When cell phone providers decided to set up a system for transmitting short messages, this channel was easy to use. All that was necessary was the addition of a Short Message Service Center to receive messages and route them to appropriate towers at an opportune time.

The system was designed by an organization called the Global System for Mobile Communications (GSM), chaired by Friedhelm Hillebrand. GSM informally analyzed the types of messages that people send to each other and concluded that people would find a message service useful even if it only transmitted between 100 and 200 characters. Restricted by the hardware to a limit of 140 bytes, or 1120 bits, they designed a system that could encode a small number of characters with 7 bits each. This yielded a maximum transmission size of 160 characters.

Other factors can lower the limit. Users can send longer messages by concatenating a group of SMS packets. However, the limit on these individual packets is reduced to 153, since each must also contain metadata describing its relationship to the other components of the larger message. SMS users who wish to communicate with other alphabets also are limited to fewer characters. Chinese, Arabic, and Cyrillic SMS use UTF-16 encoding, which requires 16 bits per character and thus has an SMS limit of 70 characters per message. Sending concatenated messages using UTF-16 encoding decreases this SMS limit to 67 characters.

The 160-character SMS limit has become widely known, and many different organizations are designing ways to communicate within this constraint. The popular messaging service Twitter is set up to conform to this limit. Twitter allows users to post a maximum of 140 characters, with the remaining space taken up by user names. Many news outlets and politicians release short messages ready to be transmitted through Twitter or sent directly to subscribers' cell phones. New services provide different ways of sending more information between cell phones, but the 160-character SMS limit remains ubiquitous.