Due to its origins on IBM Selectric-based teleprinters, 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 the IBM 2741 and IBM 1050 fitted with a specific APL print head. Over time, with the universal use of high-quality graphic display, printing devices and Unicode support, the APL character font problem has largely been eliminated.
EBCDIC code pages
includes APL symbols, in addition to preserving the basic Latin letters and Western Arabic numerals at their usual EBCDIC locations. Code page 310 includes a larger gamut of symbols, but does not itself include the basic Latin letters or the basic digits. It is used alongside Code page 037, with the Code page 310 codes being prefixed by the Graphic Escape control character. Code page 351 contains most of these additional symbols in addition to the letters and digits, by replacing several control characters with symbols.
Other pre-Unicode code pages
is an 8-bit extended ASCII code page intended for use with APL. ISO-IR-68 is a 7-bit heavily modified ASCII intended for use with APL in an environment allowing overstriking of characters.
Unicode
Most APL symbols are present in Unicode, in the Miscellaneous Technical range, although some APL products may not yet feature Unicode, and some APL symbols may be unused or unavailable in a given vendor's implementation. Missing from Unicode are the traditional underscored alphabetic characters; their usage has been eliminated or deprecated in most APL implementations. 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 specific input 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.
Keyboard layout
Note the mnemonics associating an APL character with a letter: on, on, on, on, on, on 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 the IBM 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 by overstriking one character with another. For example, the log symbol was formed by overstriking with. 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 an APL 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.
