APL

APL is a programming language with its own special character set combining Greek letters, math symbols, and more. A number of APL characters were originally produced by overstriking characters from the base set (originally 88 characters on Selectric typeballs, later extended to 95 in the APL-ASCII overlay mappings). In later APL software, there is support for typing the previously overstruck characters as single characters, typically using the Control or Alt key as a modifier. All APL symbols are included in Unicode, but not in a single block.

The A+ dialect of APL is available in most Linux distributions. It includes its own APL font.

There is no complete standard APL keyboard. Most vendors use either the typewriter-paired or bit-paired overlay layout for the basic symbols, but there is no agreement whatever on the locations of any composite symbols that are provided without overstriking.

APL is a highly mathematical language, which gives it limited appeal in spite of its power. It has been applied to almost everything in computing except writing device drivers.