A Back to Back, as this photo below, has a female on each side and allows you to make a seam between two HDMI cables.
However, attach HDMI cables was not the reason for the purchase of this device. This back to back was purchased to connect my HDMI Android away from tv, extended by a cable.
In the picture below I have the HDMI Android connected directly to the TV, What can leave the weak WiFi signal and still gets too hot the back of the TV. With this Back to Back I can connect the HDMI cable to an Android and put it in a position to better wireless network signal.
If you have any of these Google TV box, This adapter is a good idea.
Comes in a small plastic bag and ran to first. Seemed a little hard to connect the HDMI cable, but didn't cause any damage.
Those interested can follow the link below, the store DealExtreme: