Overstrike

An overstrike refers to the practice of typing glyphs on top of each other to form a more complex glyph. This was the normal way of typing many APL characters in the early typewriter terminal days of APL. Only a single shift state was available (which would switch from numbers and uppercase letters to APL glyphs), so for example  which is today typed using APL + Shift + 8 would instead be typed using APL + O for   followed by ⟵ Backspace and then  APL + P for.

Support
Support for various overstrikes vary.

APL\360
APL\360 supported many overstrikes, and these were the only way to type composite glyphs. Since typewriters couldn't remove typed characters, editing could be cumbersome, and so some innovative overstrikes were allowed for the odd case where one was correcting a typographical error. For example,  and   would form.

Dyalog APL
Dyalog APL supports the following overstrikes in any order of input: