IATA vs. ICAO

[Xyn_Air]

I did some reading about IATA vs. ICAO airport codes, and I wanted to check with everyone to see if I am understanding the general idea correctly.

IATA:
- Three-letter code, with some duplication.
- Primarily used by the 'public' (i.e. passengers) when making flight reservations and tracking luggage.
- Being phased out of use in some countries, such as Canada.

ICAO:
- Four-letter alpha-numeric code, regionally based, no duplication.
- Used by flight crew and ATC for flight planning and other such activities.

About the IATA code, I checked the reservations web pages for Northwest Airlines, Delta, and Alaska Airlines (just the three that came to me off the top of my head), and they all used the three-letter IATA airport codes to make reservations.

Obviously, in Flight Simulator the four-letter ICAO codes are used, as we are simulating acting as flight crew or ATC.  (Yes, I know this is the Real Aviation forum; I was just making a general observation  :-[ )

Now, in addition to anyone willing to tell me whether my understanding is correct, can someone explain to me why both systems are still in use?  I mean, I don't see any immediate reason why passengers couldn't use the ICAO code to make reservations (most reservation systems have an automated look-up to assist customers anyway).  Is there some significance to having both systems?

Thanks for you help and comments!

EDIT:

Upon further research, it seems the three-letter codes originate in the U.S., and that the four-letter codes for the U.S. are made simply by adding the prefix 'K' to their three-letter code (some military-related aviation activities seem to be an exception and only use three-letter codes ever).  Could the continued use of three-letter codes for airports be the result of U.S. socio-political might above other considerations (not a negative judgment of the U.S. or its place on the world stage, but merely a thought and open question)?

And then I just learned there are also five-letter codes . . . good grief!

[bok269]

You have it right.  As for why the public is expected to use the IATA codes, your guess is as good as mine.  it may have something to do with the fact that the IATA codes are more familiar to passengers?  Or because ICAO codes have region and country indicated with the first two letters, and that may be hard to memorize?

[ATI_7500]

If you were passengers and booked a flight from Hamburg to London/Stansted, you could make more sense of HAM and STN than of EDDH and EGSS, right?

I like the IATA codes. They make more sense than the ICAOs.

[bok269]

I like the IATA codes. They make more sense than the ICAOs.

It Depends.  ICAO is arranged by geography,but are very loosely based on names.  ANd keep in mind many ICAO codes make no sense at all.  ORD, MCO, PHF, IAH, IAD, DCA, etc.