The ExtensionBlock is an add-on board for the Raspberry Pi and it is a three-way GPIO extension board for the 2×20 pin header.
You stack the ExtensionBlock on top of the existing 2×20 pin GPIO header. At its corners the board provides holes for spacers, locking posts, or other tightening setups.
The ExtensionBlock splits the original 2×20 pins of the Raspberry GPIO header to three 2×20 pin headers. The headers are located such that it allows you to stack two full-size HATs side-by-side onto the Raspberry Pi.