What is Round-Trip Compatibility?
In order to get people to use Unicode, it had to work well with existing character sets. Specifically, it had to be possible to convert text in a legacy character set to Unicode and back without losing any information. In other words, Unicode had to have round-trip compatibility with all the major existing character sets. Unfortunately, the need for round-trip compatibility often means that the quirks of every existing character set are perpetuated in a new set. For instance, to have round-trip compatibility, the new character set must have the following property: for each character in a given legacy set, there must be a character in the new set that represents that legacy character and no other character from that legacy set. This often makes unification very difficult. Suppose the legacy character set has two slightly different ‘a’ characters. You would like to unify all the ‘a’s into one single ‘a’ in your new character set, but you won’t be able to, because when converting back to
