Unicode is a superset of numerous other encodings, usually the national standards of the countries whose written languages they want to encode. For example, the sections for the Greek and Cyrillic alphabets have identical layouts to existing Greek-only and Russian-only encodings.

The exceptions to this policy are usually writing systems that didn't have standardized encodings at the time they were incorporated into Unicode. Also, the rather notorious CJK unification project resulted in several new overlapping layouts as Unicode policy changed over time with respect to Chinese characters.