PICMG is a consortium of companies who collaboratively develop open specifications for high performance telecommunications and industrial computing applications.
Specs and Standards for Advanced Mezzanine Card, AdvancedTCA, COM Express, CompactPCI, e-PCI-X, MicroTCA and others.
(Study this page first to understand how computers became more maintanable and expandable - delabs)
"The PCI-ISA passive backplane standard moves all of the components normally located on the motherboard to a single plug-in card. Looking much like a standard ISA or PCI card, a PCI-ISA CPU card has two edge connectors on it - one for the PCI bus and one for the ISA bus. The "motherboard" is replaced with a simple "passive backplane" that has only connectors soldered to it."