Square serif fonts have chopped off, blocky-looking serifs that can be easily formed with the squared end of a sign painter's brush. Square serif fonts came into wide use as advertising fonts in the 1700s. Before then, few people were able to read, so advertising used very little print.
Clarendon is one of the early square serif fonts. It became known as an an "Egyptian" font in the 1800s because early promoters used square serifs like Clarendon to advertise exhibitions of the early finds from the tombs of Egypt.
Fonts with heavy, square serifs, like P.T. Barnum, were also referred to as "Circus Fonts" because they were the basis for the lettering of tent shows, circus wagons, and traveling medicine shows. While many of Barnum's circus wagons themselves had fancifully carved wodden signs and not lettered using a brush, this lettering did appear on handbills, sandwich boards, and other kinds of advertising.
Cheltenham is an exception to the "circus font" feel of square serifs and is still used for magazine work even today. It has a solid feel that sets close (that is, it puts a lot of letters on a line) and yet has an elegant proportion between x-height and ascender. Cheltenham comes in a whole variety of flavors including book, condensed, and light versions.
