Why do countries have two letter abbreviations?

In which system were countries assigned 3-letter abbreviations and which system is now using 2? Is this a Postal code you’re referring to, or something else?

I am mostly familiar with the oval bumper-stickers in Europe that have 1, 2 or 3 letters (e.g. D for Deutschland, DE for Denmark; F for France; FIN for Finland; etc.)
http://en.wikipedia.org/wiki/List_of_international_license_plate_codes

UK: United Kingdom, is just easier for official use, such as signs and the like.

i think you will find that these abreviations are used in places where countries need to be represented with a short amount of text. license plates, international sporting events and so on.

the reason it is probably two is simply the nature of abreviation, to make something as short as possible. i imagine it was originally 3 or 4 because two was not enought to make it self evident what country was being talked about.

for example, UK could stand for United Kingdom or Ukrain. leading to a need for clarification and ultimately leading to UKGB being used for the united kingdom of great britain and UKR being used for Ukrain. i would guess that after some time, some official group such as the olympic board or internation aviation union probably had a meeting to crate a official set that worked with just two initials.

this is all just a guess of course, but it makes sense to me.

You mean like the Internet country codes, the ones that end up in URLs, such as .de and .cn? I suppose because that’s the fewest characters that could be both meaningful and efficient. Here’s a list.