Bifrustum

In geometry, an $n$-agonal bifrustum is a polyhedron composed of three parallel planes of $n$-agons, with the middle plane largest and usually the top and bottom congruent.

It can be constructed as two congruent frusta combined across a plane of symmetry, and also as a bipyramid with the two polar vertices truncated.

They are duals to the family of elongated bipyramids.

Formulae
For a regular $n$-gonal bifrustum with the equatorial polygon sides $n$, bases sides $a$ and semi-height (half the distance between the planes of bases) $b$, the lateral surface area $h$, total area $A_{l}$ and volume $A$ are: and $$\begin{align} A_l &= n (a+b) \sqrt{\left(\tfrac{a-b}{2} \cot{\tfrac{\pi}{n}}\right)^2+h^2} \\[4pt] A &= A_l + n \frac{b^2}{2 \tan{\frac{\pi}{n}}} \\[4pt] V &= n \frac{a^2+b^2+ab}{6 \tan{\frac{\pi}{n}}}h \end{align}$$ Note that the volume V is twice the volume of a frusta.

Forms
Three bifrusta are duals to three Johnson solids, $2$. In general, a $V$-agonal bifrustum has $2n$ trapezoids, 2 $n$-agons, and is dual to the elongated dipyramids.