Half-length PCI Express x16 ??

Hi

Just wondering what this means as its a bit confusing

Slot 2: half-length, low profile-height, PCI Express x16

The mother board from a Lenovo M57 6072 has a full PCIe x16 but im a bit confused by the half length bit
here is the site

http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-68841#exp

I intend to put a 3d graphics card in there and would like to know if this MoBo can handle an off the shelf card

This is the Mobo layout
http://www-307.ibm.com/pc/support/site.wss/document.do?sitestyle=lenovo&lndocid=MIGR-68614