Recently, I was interested in knowing how many "primary" planes (made of coordinate axes) are in each dimension.
In 0D, there are no axes, and no planes.
In 1D, there is 1 axis (x), and no planes.
In 2D, there are 2 axes (x,y) and there is only
1 plane (xy).
In 3D, there are 3 axes (x,y,z) and there are
3 planes (xy, xz, yz).
In 4D, there are 4 axes (x,y,z,w) and there are
6 planes, (xy, xz, xw, yz, yw, zw).
In 5D, there are 5 axes (x,y,z,w,u) and there are
10 planes (xy, xz, xw, xu, yz, yw, yu, zw, zu, wu).
In 6D, there are 6 axes (x,y,z,w,u,v) and there
15 planes (xy, xz, xw, xu, yz, yw, yu, zw, zu, wu, xv, yv, zv, wv, uv).
...and so on.
What does the sequence 1, 3, 6, 10, 15 represent?
The triangular numbers!
Now, let's see how many "primary" 3D hyperplanes are in each dimension.
In 0D, there are no axes, and no 3D hyperplanes.
In 1D, there is 1 axis (x), and no 3D hyperplanes.
In 2D, there are 2 axes (x,y) and no 3D hyperplanes.
In 3D, there are 3 axes (x,y,z) and there is only
1 3D hyperplane (xyz).
In 4D, there are 4 axes (x,y,z,w) and there are
4 3D hyperplanes (xyz, xzw, xyw, yzw).
In 5D, there are 5 axes (x,y,z,w,u) and there are
10 3D hyperplanes (xyz, xyw, xyu, xzw, xzu, xwu, yzw, yzu, ywu, wzu).
...and so on.
The sequence 1,4,10,... represents
the tetrahedral numbers!
If we continue like this (finding the number of 4D-hyperplanes in n-dimensions, finding the number of 5D-hyperplanes in n-dimensions, etc.), we would obtain Pascal's Triangle! For example, if we want to know how many 4D-hyperplanes are in 6D, we have to calculate "6 choose 4", which means we are interested in knowing how many groups of 4 we can make out of 6 distinct items without considering the order of the elements in each group, i.e. "xyzw" is considered the same 4D-hyperplane as "yzwx", "zwyx", etc.
As you can see, even our Cartesian coordinate system has "triangular" properties!