Hubs that can be placed above one another on a rack and connected using special short cables to effectively form a single hub with a number of ports. Other devices, such as Ethernet switches, can also be stackable. Different vendors use different methods of stacking hubs, but they generally use some form of ribbon cable.
This is superior to the older way of cascading hubs together using the uplink port, which tends to produce crosstalk.
If you stack several hubs, the top and bottom hubs usually have a free connection that must be terminated in order to function properly.
Graphic S-17. Stackable hubs in rack.