The first thing to understand is that the xelatex engine is much more capable of employing modern fonts. was built in the late 1970's when computer resources were at a premium, and the idea of mixing mathematics with non-Western languages and scripts may have been fanciful. The pdflatex engine is rooted in this history. We now have the Unicode standard, thoroughly integrated into web browsers, and companion scalable OpenType fonts. In contrast to , XeTeX was designed to work better with a multitude of fonts. So we organize this section by this distinction.