The programming languageAPL usesa number of symbols, rather than words from natural language, to identify operations, similarly tomathematical symbols. Prior to the wide adoption ofUnicode, a number of special-purposeEBCDIC and non-EBCDICcode pages were used to represent the symbols required for writing APL.

Due to its origins onIBMSelectric-basedteleprinters, APL symbols have traditionally been represented on the wire using a unique, non-standard character set. In the 1960s and 1970s, few terminal devices existed which could reproduce them, the most popular ones being theIBM 2741 andIBM 1050 fitted with a specific APL print head. Over time, with the universal use of high-quality graphic display, printing devices andUnicode support, the APL character font problem has largely been eliminated.

IBM assigns the following character IDs (GCGIDs) to APL syntax, which are used in the definitions of its code pages.^[1][2][3]

Code page 293 (CCSID 293),^[20] called "APL (USA)", is an EBCDIC code page which includes APL symbols, in addition to preserving thebasic Latin letters andWestern Arabic numerals at their usual EBCDIC locations.^[17][18]

Code page 310 ("Graphic Escape APL/TN") includes a larger gamut of symbols, but does not itself include the basic Latin letters or the basic digits.^[22][4] It is used alongsideCode page 37-2,^[23] with the Code page 310 codes being prefixed by the Graphic Escape (EBCDIC 0x08)^[24] control character.^[6][25]

Code page 351 ("GDDM Default (USA)")^[27] contains most of the characters ofCode page 293 andCode page 310 (except⍷, epsilon with underline) in addition to the letters and digits, by replacing severalcontrol characters with symbols.

Code page 371,^[28] registered for use withISO/IEC 2022 asISO-IR-68,^[29][5] is a 7-bit heavily modified ASCII, designed by the APL Working Group of theCanadian Standards Association, intended for use with APL in an environment allowing overstriking of characters using theBS (backspace, 0x08) control code.^[29][5]

Code page 907 is used by the IBM 3812, like code page 906.

Code page 909 is another encoding for APL, differing from code page 907 in not including the underlined characters, assigning different codes to the APL characters which fall in the 0xB0–DF range, and replacing some of the C0 replacement graphics from code page 437 with alternative encodings for certain APL symbols.

Code page 910 is similar to code page 909, but with fewer duplicate horizontal arrows, using the same C0 graphics as code page 437, and including some additional characters.

Most APL symbols are present in Unicode, in theMiscellaneous Technical range,^[30] although some APL products may not yet feature Unicode, and some APL symbols may be unused or unavailable in a given vendor's implementation.

As of 2010, Unicode allows APL to be stored in text files, published in print and on the web, and shared through email and instant messaging. Entering APL characters still requires the use of either a specificinput method editor or keyboard mapping, or of a specific touch interface. APL keyboard mappings are available for free for the most common operating systems, or can be obtained by adding the Unicode APL symbols to existing keyboard map.

Missing from Unicode are the traditionalunderscored alphabetic characters included in some of the APL code pages; their usage has been eliminated or deprecated in most APL implementations. These were produced on APL printing terminals by over-striking a straight capital letter with an underscore character. Some tables show them simulated with underlined and italic markup, not listing Unicode mappings.^[4]

IBM assigns them GCGIDs as "LA480000" (which they name "A Line Below Capital/A Underscore (APL)"), "LB480000" ("B Line Below Capital/B Underscore (APL)") and so forth, under the "L" series used for Latin letters.^[1] The use of an even number (48) rather than an odd number (47) is due to being uppercase: compare the use of SD110000 for a lone acute accent´, LA110000 for the lowercaseá, and LA120000 for the uppercaseÁ.^[31] They are included in IBM'sprivate use area scheme, encoded in reverse‑alphabetical order in the odd-numbered code points from U+F8BF to U+F8F1.^[12]

Homologous uses of 47 include the "SD" (diacritic) series GCGID SD470000 for "Line Below/Discontinuous Underscore"^[32]—i.e.macron below, distinct from the ASCII underscore which is SP090000 ("Underline/Continuous Underscore")^[31]—and the "A" (Arabic letter) series GCGID AD470009 for theḏāl,^[33] for example. Unicode'sLatin Extended Additional block includes the following capital "Line Below" characters with the macron below diacritic, for Semitic transcription (it includes a pre-composed ẖ only in lowercase):

• U+1E06 ḆLATIN CAPITAL LETTER B WITH LINE BELOW
• U+1E0E ḎLATIN CAPITAL LETTER D WITH LINE BELOW
• U+1E34 ḴLATIN CAPITAL LETTER K WITH LINE BELOW
• U+1E3A ḺLATIN CAPITAL LETTER L WITH LINE BELOW
• U+1E48 ṈLATIN CAPITAL LETTER N WITH LINE BELOW
• U+1E5E ṞLATIN CAPITAL LETTER R WITH LINE BELOW
• U+1E6E ṮLATIN CAPITAL LETTER T WITH LINE BELOW
• U+1E94 ẔLATIN CAPITAL LETTER Z WITH LINE BELOW

However, this does not cover the entireISO basic Latin alphabet, and IBM's reference glyphs for the APL characters show them both underlined andoblique,^[2] and tables simulating them with markup may follow suit.^[4] Unicode'sMathematical Alphanumeric Symbols block includes italic characters for use in notations where they are contrastive with non-italic characters. Unicode also includes combining forms of the macron below and underscore in theCombining Diacritical Marks block; the characters above canonically decompose with the former:

• U+0331 ◌̱COMBINING MACRON BELOW
• U+0332 ◌̲COMBINING LOW LINE

Note the mnemonics associating an APL character with a letter:? (question mark) onQ,⋆ (power) onP,ρ (rho) onR,⊥ (base value) onB,⊤ (eNcode) onN,∣ (modulus) onM and so on. This makes it easier for an English-language speaker to type APL on a non-APL keyboard, providing one has visual feedback on one's screen. Also, decals have been produced for attachment to standard keyboards, either on the front of the keys or on the top of them.

Later IBM terminals, notably theIBM 3270 display stations, had an alternate keyboard arrangement which is the basis for some of the modern APL keyboard layouts in use today.

Further APL characters were available byoverstriking one character with another. For example, thelog symbol (⍟) was formed by overstriking⇧ Shift+P with⇧ Shift+O. This extended the graphic abilities of the earlier teleprinters, but made it more complex to correct errors and edit program lines.

New overstrikes were introduced by vendors as they produced versions of APL tailored to specific hardware, system features, file systems, and so on. Further, printing terminals and early APL cathode-ray terminals were able to display arbitrary overstrikes, but as personal computers rapidly replaced terminals as a data-entry device, APL character support became provided as anAPL Character Generator ROM or a soft character set rendered by the display device. With the advent of the modern PC, APL characters were defined in specific fonts, eliminating the distinction between overstruck characters and standard characters.

Finally, the symbols were ratified in Unicode and given specific code points, with unambiguous interpretations, independently of the graphic font.

• APL syntax and symbols
• ISO IR-68

• IBM code page 293 a.k.a. the APL code page onmainframe computers
• IBM code page 907 a.k.a. the APL ASCII code page

• APL programming language family
• Character sets

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