APL code page
The APL code page was used in early implementations of APL on IBM computer systems and terminals. It was an EBCDIC-based character set which replaced many of the characters with the specialized ones needed by the APL programming language. The original mainframe version was IBM code page 293, while a later ASCII-based version was IBM code page 907.
"Classic" APL character sets have only uppercase letters in them (normally presented italicized), though newer ASCII- or Unicode-based sets generally have lowercase available as well, among other characters not part of "purist" APL.
Internally to APL, characters are assigned code positions within what is known as the "Atomic Vector" (⎕IO), which can differ between systems; one such chart can be found starting on page 201 of the IBM 5100 APL manual (page 207 of this PDF version).
Traditionally, some APL characters with distinct meanings were formed by backspacing and overstriking other characters (originally on a printing terminal). For instance, the "quote quad" is formed by overstriking the "quad" (a rectangular shape) with a single quote. These composite characters now have separate code positions in the Unicode set.