Feature: Overstruck Characters
Way back, there were typewriters. To keep them simple, sometimes they omitted certain characters. In the example below, there is no “1” key or “0” (zero) key — because to make those symbols, you can just “I” and “O.” On a keyboard, it is just ink, it isn’t digital data.
The idea of “overstruck characters” is typing one character on top of another, with the resulting combination being a new 3rd character. Like old typewriters, originally on computers the backspace did not automatically erase the previous character. So on the old mechanical typewriters, you could type the letter O (which was stamped by ink and couldn’t be undone), then backspace and type the letter I. The result was then the Greek letter called phi:
The IBM 5100 has a similar feature. While the feature is only explained in the APL manual (Appendix B, where it identifies the feature as “Overstruck characters”), the feature works just the same on the BASIC-only models. As a prime example, the exclamation mark is not a single key on the keyboard. It must be entered using an overstruck sequence of two characters.
To the IBM 5100, “!” is an APL function of Factorial (as appropriate to a more scientific focused machine). But for a desktop publishing system, “!” would be an exclamation mark.