Historically many of the APL glyphs were produced via digraphs - 'quad' is [] overstruck, making a narrow rectangle. The rotation verbs overstrike a circle with

Historically many of the APL glyphs were produced via digraphs - 'quad' is [] overstruck, making a narrow rectangle. The rotation verbs overstrike a circle with -, |, or \. While the original character set was different* , it seems like a reasonable alternative to a completely different keyboard layout (in hardware or software). * According to the APL book I was looking through recently, some keyboards required the user to overstrike ' and . to get an ! (!).