Glyph

Glyphs are the individual characters and symbols used in APL, primarily to represent primitive functions and operators. A glyph is distinct from the functionality it represents, and usually has a different name: for example, the dyadic function Take is represented with the glyph Up Arrow.

Most of APL's glyphs cannot be represented in ASCII, now considered a "standard" or "basic" character set, but all of them are included in Unicode, as one goal of Unicode was to unify existing character sets. In fact, Ken Iverson first began using his notation before work on ASCII began, and APL\360 was implemented before the modern ASCII standard was published in 1967. Prior to the widespread adoption of Unicode, many special purpose code pages were used to represent APL symbols. APLs developed more recently, such as NARS2000 and GNU APL, use Unicode characters to represent glyphs, sometimes supporting several different options for a given glyph. Unicode also offers the possibility of introducing glyphs that could not have been produced on older APL systems: for example, both NARS2000 and dzaima/APL use  for the Square Root and Root functions.

Because of difficulties associated with non-ASCII characters (while display problems have been almost eliminated by Unicode, entering the characters may still be a barrier), several array-family languages such as J, K, and ELI have chosen to encode the language using only ASCII, either by reducing and compacting functionality to use one character per glyph or by using multiple characters in some cases (in J, these are called "bigraphs" and "trigraphs"). Historically there have also been various encodings of APL in smaller character sets, typically as an alternate way of writing code for an APL with traditional glyphs.

While Iverson notation was originally handwritten, the choice of glyphs to include in the first APL implementations was influenced by technical constraints of the typewriters used at the time. Notably, many glyphs were produced by overlaying two simpler glyphs, a technique known as overstriking.