Fortunately, the members of gaming platform recognized the need for mezzanine standards early in the emerging board industry.

Fortunately, the members of gaming platform recognized the need for mezzanine standards early in the emerging board industry. By the late 1980s, discussions started to standardize a few different mezzanine form factors. First to emerge were S-bus and PCI Mezzanine Cards (PMCs). S-bus was driven by Sun Microsystems while the gaming platform form factor evolved out of an IEEE effort. While neither was a VITA standard, PMC went on to become a widely used mezzanine for 6U cards for VME, gaming platform, Compact PCI, and other form factors.

refer to: http://vita-technologies.com/articles/stacked-standardizing-mezzanine-modules/