Generally: it's a way to tweak a font family for use in a specific design context. Obvious parameters would be weight and width, but others could be imagined as

Generally: it's a way to tweak a font family for use in a specific design context. Obvious parameters would be weight and width, but others could be imagined as well. In the case of Prototypo, it looks like you tweak things upstream, and then generate concrete font files for each variant you want; those are what you then use in your project. This still has the downside though of having to distribute multiple fonts, though, even if they're related. Looking into the future, I wonder if it will ever make sense to push that downstream, and distribute one parametric font to clients. You could then use some styling language on the client end to have the client render the variants you want...