misc-fixed

The names are cell sizes. 6x13 is what the alias fixed points at, and therefore what xterm, xclock and nearly every other core program draws with unless told otherwise; 5x7 and 5x8 were for X terminals with small screens, 9x15 and 10x20 for people sitting close to a large one. The fonts came out of MIT's X distribution in the 1980s under the foundry name misc, since nobody really owned them, and they are in the public domain. They are pure bitmaps, drawn by hand pixel by pixel, which is why they are still crisper at their size than most things that are rasterised.

In 1997 Markus Kuhn began extending them to ISO 10646 as part of the push to make Unix speak UTF-8, with volunteers drawing glyphs for Cyrillic, Greek, Hebrew, Armenian, mathematics and assorted symbols. The ucs-fonts package went into XFree86 4.0 in 2000, so a modern X has an iso10646-1 6x13 with thousands of characters, all in the same six-pixel-wide cell. It is still what you get when a program falls back to fixed, and one of the few things in an X installation that has not changed in forty years.