Single-letter second-level domains are domains in which the second-level domain of the domain name consists of only one letter, such as x.com. In 1993, the Internet Assigned Numbers Authority (IANA) explicitly reserved all single-letter and single-digit second-level domains under the top-level domains com, net, and org, and grandfathered those that had already been assigned. In December 2005, ICANN considered auctioning these domain names.

On December 1, 1993, the Internet Assigned Numbers Authority (IANA) explicitly reserved the remaining single-letter and single-digit domain names. The few domains that were already assigned were grandfathered in and continued to exist.^{[1][better source needed]}

The six single-letter domains in existence at that time under .com, .net and .org were the following:^[1]

The .org TLD was subsequently reopened for single-letter domain registrations. These and selected other gTLD and ccTLD single-letter domain names currently in use, typically as shortcuts, are listed below.

Many other single-letter second-level domains have been registered under country code top-level domains. The list of country code top-level domains which have been identified to allow single-letter domains are:

In 2012, the Public Interest Registry (PIR) initiated Project94, in which 94 one- and two-letter domains in the top-level domain org, that had been traditionally reserved, are awarded to qualifying organizations.^[11]

Only three of the 26 possible single-letter domains have ever been registered under the .com domain, all before 1992. The other 23 single-letter .com domain names were registered January 1, 1992 by Jon Postel,^[12] with the intention to avoid a single company commercially controlling a letter of the alphabet. Many but not all .com two-letter domain names are among the most valuable domain names.

While it is widely believed that the domain names business.com and sex.com have been the most valuable domain name transactions, prominent two-letter domain names have only been sold after nondisclosed transactions handled by specialized broker and law firms.

The value of the LG Corp (the South Korean electronics conglomerate formerly known as Lucky-Goldstar) purchase of LG.com was never published. LG Group missed the first sale of the domain name in 2008 from the original owner the chemical company Lockwood Greene to the dot-com entrepreneur Andy Booth; Booth had used it to launch a footballing website known as LifeGames. LG Corp bought "lg.com" one year later, in 2009. Following the purchase, LG Group changed worldwide marketing to LG.com, which is now their central internet address for all countries. All national LG country domain names like "LG.de" or "LG.com.mx" redirect to "LG.com".

The value of the initially secret November 2010 Facebook purchase of FB.com was revealed two months later to be $8.5 million in cash and the rest in stocks.^[13]

IG Group paid $4.7 million in September 2013 to buy IG.com.^[14]

GMO Internet, Inc. purchased Z.com for nearly $6.8 million from Nissan, who previously used it for the Nissan Z series cars.^[15]

With the 2005 announcement that registration of the remaining single-letter names might become available, some companies have attempted to establish a right to the names by claiming trademark rights over single letters used in such a context. U magazine, a college-oriented publication, went so far as to rebrand its website as "U.com" and apply for a trademark registration of the same phrase, before sending a letter to ICANN attempting to gain priority for the domain if it should ever become available in the future.^[16]