A 2D-Pyramid is a simple-to-build yet aesthetically pleasing structure that can be toppled by removing just a single domino on either side.

Building a 2D-Pyramid involves repeating two different types of layers as shown below. If you are unsure where to place the dominoes for the next layer, look down for the largest vertical gaps without dominoes. These should be bridged next.

If b is the number of dominoes touching the ground, the entire 2D-Pyramid will require $$\frac{b \cdot (b+1)}{2}$$ dominoes.