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DEC provided an APL softchar font for the VT340 that allowed programmers to write in the APL language without having to use one of those fancy IBM selectric typewriters with a special "golf ball" typing element to handle all the strange characters.
The APL programming language imposes requirements on a terminal similar to a foreign natural language. APL uses,
- a different nomenclature for describing the characters,
- a different font for output, and
- a different keyboard for input.
See the VAX APL User's Guide for information on how those pieces were defined and worked together.
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Historical footnote: Long before the VT340, DEC sold terminals for APL programming. In 1980 DEC released DECSTD-107 which defined the layout for an APL keyboard and released a specialized version of the VT102. That keyboard layout and character set was not used ten years later on the VT340.