Greco joint
From Brickwiki
The Greco joint is a SNOT technique that joins two bricks bottom to bottom.
The Greco Joint is the work of Felix Greco and needs only three parts: two bricks and as many small cross-axles as there are tubes that line up between the two bricks.
The cross-axles are wedged into the tubes, and they join the two bricks together.
Caution
This joint is difficult to undo and may damage the bricks. It is a tight fit in the tubes, and can be very hard to take apart. As well, the strain on the ABS may cause warping or breakage of the tubes.
