http://www.lucasfonts.com/fonts/thesansmono/thesansmono/overview/

TheSans Mono was designed to fulfill a need for a legible, elegant monospaced (i.e. equal-width) typeface for computer coding. Having been designed for a specific project, it was later expanded into an eight-weight, three-width family. The typeface’s three widths have been calculated to comply, at 12pt size, with the Pitch Widths customary in monospaced typewriter fonts. In this system, Pitch 12 corresponds to 12 characters per inch, Pitch 10 to 10 characters, etc.