All single octet length top level domain (TLD) names are reserved.
    Should the root zone ever get very large, there are technical
    solutions involving referral to servers providing splits of the zone
    based on the first name octet, which would be eased by having the
    single byte TLDs available.  In addition, these provide a potential
    additional axis for DNS expansion.  For like reasons, it is
    recommended that within TLD zones or indeed within any zone that is
    or might become very large, in the absence of a strong reason to the
    contrary, all single octet names be reserved.

"Before the current reserved name policy was imposed in 1993, Jon Postel (under the IANA function) took steps to reserve all available single character letters and numbers at the second level for future extensibility of the Internet (see 20 May 1994 email from Jon Postel, https://web.archive.org/web/20100301054658/http://ops.ietf.o...)"

Although I can't find off hand where he said about the corporations but I do vaguely remember reading that at some time as well, may be wrong on that specific point though.

https://web.archive.org/web/20030614022228/http://ops.ietf.o...

> 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, with the intention to avoid a single company commercially controlling a letter of the alphabet.

"Everyone else was getting rich and these guys were trying to keep the infrastructure together," Kahle said.

And perhaps chase the Wikipedia citation to see what old article it use to quote.