Wikipedia
ChiWriter is a commercial scientific text editor for MS-DOS, created by Cay Horstmann in 1986. It was one of the first WYSIWYG editors that could write mathematical formulas, even on IBM PC XT computers that were then common.
The editor was more oriented to speed and interactive aspect of editing, rather than on visual appearance of the result, therefore, it has its own graphical user interface with bitmap fonts of fixed width. Although popular (easier to use than TeX for many scientists), this unfortunately led to its demise as more text editors with vector fonts for MS Windows began to appear, and in 1996, it has been discontinued.
The basic idea of ChiWriter is that you can add lines above or below the current text line (by half of the base line height), and write text there too. The additional lines are treated as part of the original text line. This can be used to create subscripts and superscripts, and also more complex formulas such as fractions. Combined with the possibility to use several fonts (up to 20) at once, such as Greek alphabet, Cyrillic alphabets, and mathematical and other symbols, ChiWriter is quite easy to write mathematical texts in. Every font has the same fixed dimensions, but you can use different sets for different output devices (for example, you can use low-resolution fonts for screen and high-resolution fonts for printer). You can also manually create the larger objects (such as sums and integrals) by using several symbols for each part of the object.
The editor was quite customizable. There was also a font editor, which allowed to modify fonts and introduce user-designed fonts (including proportional ones) and symbols.