An obelisk is a stone pillar erected as a monument that usually consists of a pyramid atop a tall square (or sometimes rectangular base). The elongated square pyramid can therefore be considered as a special case of an obelisk.
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Harris and Stocker (1997) use the term "obelisk" to mean a polyhedron formed by two noncongruent parallel rectangles whose side faces are trapezoids. The volume of such a solid is given by
 |  | ![1/6h[(2a+a^')b+(2a^'+a)b^']](/images/equations/Obelisk/Inline3.svg) |
(1)
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 |  | ![1/6h[ab+(a+a^')(b+b^')+a^'b^'].](/images/equations/Obelisk/Inline6.svg) |
(2)
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The distance from the bottom base to the geometric centroid is
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(3)
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The term obelisk is sometimes also used to refer to the dagger symbol (Bringhurst 1997, p. 275).
