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The Tibetan encoding systems are needed for writing Tibetan fonts on a computer -- or long time ago on a typewriter. The future for Tibetan encoding is Unicode, but until then there's a need for various encoding systems.

The most classical one is Wylie Encoding, created by Professor Wylie. This is used in Tibetan books and similar places for encoding Tibetan words using plain ASCII. As an example, dharma spelled out in Wylie is chos.

A variation of this is ACIP Encoding that introduced a standard for Sanskrit Tibetan letters.

Various Tibetan fonts use different encoding schemes with multiple characters to represent Tibetan letters. These are not interchangeable, if you use for example TCC fonts you can't switch to Nitartha-Sambhota fonts, and vice versa.

Long term Unicode Tibetan will become the general standard for Tibetan letters, but this has not yet happened due to lack of good Unicode Tibetan font sets.

Note that the encoding schemes are different from the pronunciation rules. There are very few standards concerning how to spell out pronunciation, of which the ACIP Standards for Simplified Tibetan is one standard of many.