ISO/IEC 13751:2001

ISO/IEC 13751:2001, or the Extended APL standard, is an international standard specifying behavior for a nested APL dialect. Issued jointly by ISO and IEC and succeeding ISO 8485:1989 (which supported only flat and not nested arrays), it is the most recent APL standard. While based primarily on APL2, the standard includes some features not present in APL2, such as the Commute and Rank operators, the Identity functions, and GCD and LCM extending Or and And.

The Extended APL standard was drafted by a working group chaired by Lee Dickey. Work began in the late 1980s, and the first drafts, which differed little from the final design, were circulated beginning in 1993.

Some modern dialects, such as APLX and GNU APL, are based on ISO/IEC 13751:2001, while others, such as APL+Win and Dyalog, have a slightly different set of quad names and provide the migration level functionality to improve compatibility. In a 2005 study (prior to the existence of GNU APL, as well as the addition of features from the standard to various dialects), the obscure dialect APL2C was found to have the fewest features differing from the standard, followed by APLX and APL2.