Mezzanine boards are small form factor cards designed to plug onto larger form factor main boards.
The larger main boards can be designed to support one or more Mezzanine boards.
However, not all mezzanine formats are supported by all main board formats.

The XMC standard supports a number of different board sizes, the standard single wide card is shown below.
The double wide card is also supported with a width of 149mm, also with a length of 149mm.
Although it might be more common to find two individual, single-wide, mezzanine card slots on a carrier card.

Single-wide & Double-wide XMC Boards

Connectors P11 to P14 are the same 64-pin connectors defined in the CMC specification.
While two new high density connectors [J15 & J16] were added by the XMC specification.
In fact there is also a shortened board format [139mm] that drops the 64-pin connectors all together.

XMC board sizes include: Single wide 74mm x 149mm cards
Double wide 149mm x 149mm cards
Short Single wide 74mm x 139mm cards [PCI connectors omitted]
Short Double wide 149mm x 139mm cards [PCI connectors omitted]

Switched Mezzanine Card

XMC [VITA 42], Switched Mezzanine Card , is compatible with Compact PCI or the VME board standards, and ATCA carrier cards.
The XMC board standard is based on the PMC mechanical definition, and occupies the same board area.
Among the differences between XMC and PMC standards are the addition of a new set of connectors and a fabric interconnect.

The XMC board dimensions is 74mm x 149mm for single width cards, or 149mm x 149mm for double width Modules.
In addition, a card may also be defined as short [139mm] which omits the PCI connectors from the card.
The four PCI connectors P11 to P14 are the connectors on the far side of the XMC board, the two in-board connectors are high-speed.

The XMC standard adds to the PMC specification by including switched fabric interconnects to the existing PCI bus interface.
An XMC module adds one, or optionally two, new connectors to the connectors already on a PMC. The new connectors support high-speed differential signals for fabric communications.

XMC Mezzanine Card Manufacturers for XMC Board manufacturers.
Other styles of Mezzanine Card Manufacturers are also listed, as links.

The pin out of the on-board connectors change depending on the XMC standard [VITA standard]

XMC Standards

VITA 42.0 XMC: Switched Mezzanine Card Base Specification [2008]
VITA 42.1 XMC Parallel RapidIO Protocol Layer Standard, 2006 [XMC Switched Mezzanine Card: Parallel RapidIO]
VITA 42.2 XMC Serial RapidIO Protocol Layer Standard, 2006 [XMC Serial RapidIO Protocol Layer Standard]
VITA 42.3 XMC PCI Express Protocol Layer Standard, 2006
VITA 42.4 XMC HyperTransport Protocol Layer Standard [Not being worked]
VITA 42.5 XMC Aurora Pin Assignments [Not being worked]
VITA 42.6 XMC: 10 GbE Protocol Layer, 2009
VITA 42.10 XMC General Purpose I/O Standard [Not being worked]

These are all VITA Standards, indicated by the term 'VITA'.

Note the pin out tables are derived from a Mercury Computer overview and not from the standard.

Each of these mezzanine board formats are developed to handle two different issues, an electrical pinout and a physical size.
The physical size of the card is developed to match available space on a selected main board, or carrier board.
Although in many cases any new mezzanine board would comply with the Common Mezzanine Card format.
The electrical specification calls out what ever the card is designed to handle, in addition to undefined I/O.
So card formats that carry the newest electrical standards are the ones in current demand.
While the card formats that carry the older electrical specifications come to be used on legacy carrier boards.
For example the XMC card is based on the older PCI based PMC mezzanine card, but adds the newer electrical spec for PCIe.
It's also common for a new board format that contains a new electrical spec to also carry the previous generation.

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