Triagonals

In geometry a space diagonal (also interior diagonal or body diagonal) of a polyhedron is a line connecting two vertices that are not on the same face. Space diagonals contrast with face diagonals, which connect vertices on the same face (but not on the same edge) as each other.

For example, a pyramid has no space diagonals, while a cube (shown at right) or more generally a parallelepiped has four space diagonals.

An axial diagonal is a space diagonal that passes through the center of a polyhedron.

For example, in a cube with edge length a, all four space diagonals are axial diagonals, of common length ${\displaystyle a{\sqrt {3}}.}$ More generally, a cuboid with edge lengths a, b, and c has all four space diagonals axial, with common length ${\displaystyle {\sqrt {a^{2}+b^{2}+c^{2}}}.}$

...
Wikipedia