COM Express is a Computer-On-Module (COM) standard that defines several different COM form factors. The three form factors are :
Mini: 55×84 mm
Compact: 95×95 mm
Basic: 95×125 mm
Extended: 110×155 mm

In addition to these different form factors, there are currently 3 different pinouts (pin arrangements on a connector) in common use. These 3 different pinouts allow for different I/O, display and expansion functionality. The connector of all COM Express boards is on the bottom of the board. It is not an edge connector. In this way, it sits right on top of the carrier board.