Twelve South HiRise for iMac review | Monitors Reviews | TechRadar:

"The Twelve South HiRise raises the height of your iMac or Apple Display without ruining its aesthetics.
Your Apple device's L-shaped aluminium stand fits into a slot at the back of the HiRise, and rests on a height-adjustable shelf within the unit. Just find which of its six slots is the most comfortable for your viewing.
One of the slots height-aligns a Thunderbolt Display with your iMac. This is very useful if you use a Thunderbolt display as a second screen."