OpenType lets you replace characters based on context

I started out being extremely confused about what OpenType even is. I still don’t know much, but I learned that you can write extremely simple OpenType rules to change how a font looks, and you don’t even have to really understand anything about fonts.

Here’s an example rule:

sub a' b by other_a;
What sub a' b by other_a; means is: If an a glyph is before a b, then replace the a with the glyph other_a.