DB9 refers to a common connector type from the D-Subminiatures
(D-Sub) connector family, which when introduced, was among the smallest
connectors used on computer systems. DB9 houses 9 pins (for the
male connector) or 9 holes (for the female connector). DB9 connectors
were once very common on PCs and servers. Today, the DB9 has mostly
been replaced by more modern interfaces such as USB, PS/2, Firewire,
and others.