A PCI Mezzanine Card or PMC is a printed circuit board assembly manufactured to the IEEE P1386.1[1] standard.
[2] A mezzanine connector connects two parallel printed circuit boards in a stacking configuration.
In addition, arbitrary connectors can be supplied on the front panel of the chassis or case; also known as a "bezel".
The PMC standard defines which connector pins are used for which PCI signals; in addition it defines the optional 64 "P4" connector pins for use of arbitrary I/O signals.
PMCs were also used in early ATCA systems prior to the advent of the Advanced Mezzanine Card or AMC.