In geometry, a parallelepiped is a three-dimensional figure formed by six parallelograms (the term rhomboid is also sometimes used with this meaning). By analogy, it relates to a parallelogram just as a cube relates to a square. In Euclidean geometry, the four concepts—parallelepiped and cube in three dimensions, parallelogram and square in two dimensions—are defined, but in the context of a more general affine geometry, in which angles are not differentiated, only parallelograms and parallelepipeds exist. Three equivalent definitions of parallelepiped are
"Parallelepiped" is now usually pronounced /ˌpærəlɛlɪˈpɪpɛd/, /ˌpærəlɛlɪˈpaɪpɛd/, or /-pɪd/; traditionally it was /ˌpærəlɛlˈɛpɪpɛd/PARR-ə-lel-EP-i-ped in accordance with its etymology in Greek παραλληλεπίπεδον parallelepipedon, a body "having parallel planes".
Any of the three pairs of parallel faces can be viewed as the base planes of the prism. A parallelepiped has three sets of four parallel edges; the edges within each set are of equal length.
Parallelepipeds result from linear transformations of a cube (for the non-degenerate cases: the bijective linear transformations).
Since each face has point symmetry, a parallelepiped is a zonohedron. Also the whole parallelepiped has point symmetry Ci (see also triclinic). Each face is, seen from the outside, the mirror image of the opposite face. The faces are in general chiral, but the parallelepiped is not.
The parallelepiped with Oh symmetry is known as a cube, which has six congruent square faces.
The parallelepiped with D4h symmetry is known as a square cuboid, which has two square faces and four congruent rectangular faces.
The parallelepiped with D3d symmetry is known as a trigonal trapezohedron, which has six congruent rhombic faces (also called an isohedral rhombohedron).
For parallelepipeds with D2h symmetry, there are two cases:
Rectangular cuboid: it has six rectangular faces (also called a rectangular parallelepiped, or sometimes simply a cuboid).
Right rhombic prism: it has two rhombic faces and four congruent rectangular faces.
Note: the fully rhombic special case, with two rhombic faces and four congruent square faces , has the same name, and the same symmetry group (D2h , order 8).
For parallelepipeds with C2h symmetry, there are two cases:
Right parallelogrammic prism: it has four rectangular faces and two parallelogrammic faces.
Oblique rhombic prism: it has two rhombic faces, while of the other faces, two adjacent ones are equal and the other two also (the two pairs are each other's mirror image).
A perfect parallelepiped is a parallelepiped with integer-length edges, face diagonals, and space diagonals. In 2009, dozens of perfect parallelepipeds were shown to exist, answering an open question of Richard Guy. One example has edges 271, 106, and 103, minor face diagonals 101, 266, and 255, major face diagonals 183, 312, and 323, and space diagonals 374, 300, 278, and 272.
Some perfect parallelepipeds having two rectangular faces are known. But it is not known whether there exist any with all faces rectangular; such a case would be called a perfect cuboid.
Coxeter called the generalization of a parallelepiped in higher dimensions a parallelotope. In modern literature expression parallelepiped is often used in higher (or arbitrary finite) dimensions as well.
Specifically in n-dimensional space it is called n-dimensional parallelotope, or simply n-parallelotope (or n-parallelepiped). Thus a parallelogram is a 2-parallelotope and a parallelepiped is a 3-parallelotope.
More generally a parallelotope, or voronoi parallelotope, has parallel and congruent opposite facets. So a 2-parallelotope is a parallelogon which can also include certain hexagons, and a 3-parallelotope is a parallelohedron, including 5 types of polyhedra.
The edges radiating from one vertex of a k-parallelotope form a k-frame of the vector space, and the parallelotope can be recovered from these vectors, by taking linear combinations of the vectors, with weights between 0 and 1.
The n-volume of an n-parallelotope embedded in where can be computed by means of the Gram determinant. Alternatively, the volume is the norm of the exterior product of the vectors:
If m = n, this amounts to the absolute value of the determinant of the n vectors.
Another formula to compute the volume of an n-parallelotope P in , whose n + 1 vertices are , is
where is the row vector formed by the concatenation of and 1. Indeed, the determinant is unchanged if is subtracted from (i > 0), and placing in the last position only changes its sign.
Similarly, the volume of any n-simplex that shares n converging edges of a parallelotope has a volume equal to one 1/n! of the volume of that parallelotope.
Charles Hutton's Dictionary (1795) shows parallelopiped and parallelopipedon, showing the influence of the combining form parallelo-, as if the second element were pipedon rather than epipedon. Noah Webster (1806) includes the spelling parallelopiped. The 1989 edition of the Oxford English Dictionary describes parallelopiped (and parallelipiped) explicitly as incorrect forms, but these are listed without comment in the 2004 edition, and only pronunciations with the emphasis on the fifth syllable pi (/paɪ/) are given.
A change away from the traditional pronunciation has hidden the different partition suggested by the Greek roots, with epi- ("on") and pedon ("ground") combining to give epiped, a flat "plane". Thus the faces of a parallelepiped are planar, with opposite faces being parallel.