A special-purpose font that demonstrates the relationships between variable “width” axes for use with different writing directions. For example, horizontal layout is typical with Western characters (represented as hollow rectangles in this font), but vertical layout is often used with some Asian characters (represented as solid rectangles), and it’s possible to combine aspects of both in different ways. This can make the relationship between different glyphs’ width, height, and rotation complicated. Ken Lunde explains more in the article introducing the font.