Here's the answer to a question I realize that I should have asked years ago. "Why 'EE. UU.' for 'Estados Unidos'?" It was asked and answered by Spanish.about.com.
The double E and double U indicate that the letter represents a plural. Some other common Spanish abbreviations, among them FF. AA. for Fuerzas Armadas (Armed Forces) and AA. EE. for Asuntos Exteriores (Foreign Affairs), do the same thing. (Also in very common use are the abbreviations without the spaces and/or periods, such as EEUU, FFAA and AAEE.) Such a doubling of letters isn't done for all plurals; ONU is the abbreviation for la Organización de las Naciones Unidas, the United Nations.
We do the same doubling of letters in English in a few cases for words of Latin origin. For example, the abbreviation for "page" is "p.," while for "pages" it's "pp." (The same abbreviations are used in Spanish for página and páginas.) And the abbreviation for "manuscript" is "MS" or "ms," while in the plural it's "MSS" or "mss." (Again, the same abbreviations are used in Spanish.)
You will in Spanish occasionally see the abbreviations EUA (for Estados Unidos de América) and even USA for Estados Unidos, but much less commonly than EE. UU. and its variations.
And given the number of times in my life that I have written or typed "pp.", I'm embarrassed that I never had a conscious thought as to why.
did ask why the United Nations (Organización de las Naciones Unidas) is abbreviated as ONU in Spanish, rather than ONNUU. Buena pregunta (good question). Does anyone have an answer?
I got three responses to this question -- my record.
1) Actually, NNUU is used, it's just not as common as ONU. Saludos, Susan Greenblatt
2) Here is my understanding about "The United Nations." It is an international organization, with a high degree of standardization, and six official languages:
So, in an effort to standardize the name of the organization, we have
1. English: UNO (United Nations Organization, even though the shorter version, UN is commonly used)
2. French: ONU (Organization des Nations Unies)
3. Spanish: ONU (Organizición de las Naciones Unidas).
Saludos, Donato Fhunsu
3) My guess: maybe because organizacion is singular. Estados and Fuerzas are both plural. Sonna Loewenthal