A Simplex can be described as the 4-space member of a class of regular pyramids.  In general these pyramids are formed by selecting a point in the next higher space that is equidistant from all the vertices in the pyramid of the current space and connecting that point to all the vertices of the current vertices.  The table below describes the characteristics of pyramids in the first 4 dimensions.
Image
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Description
Point
1
Beginning with 0 space, all figures may be represented by a point.
Line
2
1
In 1 space, selecting a point outside the original 0 space and connecting it to the vertex of the 0-space figure forms a line.  This pyramid has 2 vertices and a single edge.
Triangle
3
3
1
In 2 space, selecting a point outside the space of the line which is equidistant from the end points of the line and connecting it to the vertices of the line forms a triangle.  This pyramid has 3 vertices, and is enclosed by 3 edges.  (Although this figure is an equilateral triangle, its image has been skewed to match one of the faces in the next figure.)
Tetrahedron
4
6
4
1
In 3 space, selecting a point which is outside the 2 space of the triangle and equidistant from its 3 vertices and connecting it to the vertices of the triangle forms a Tetrahedron.  This pyramid has 4 vertices, 6 edges, and is enclosed by 4 triangular faces.  Again the image is skewed.
Simplex
5
10
10
5
1
In 4 space, selecting a point outside the 3 space of the tetrahedron which is equidistant from all the vertices of the tetrahedron and connecting that point to all the vertices of the Tetrahedron forms a Simplex.  This pyramid has 5 vertices, 10 edges, 10 faces, and is enclosed by 5 tetrahedra.
This progression may be extended into as may spaces as the viewer cares to.  For example a 5 space Simplex will have 6 vertices, (1 more than the number of dimensions in that figure's space). The number of edges will be the number of vertices plus the number of edges from the pyramid in the previous space (5+10=15).  The number of faces will be the number of edges plus the number of faces from the pyramid in previous space (10+10=20).  The number of volumes would be the number of faces plus the number of volumes from the pyramid in the previous space (10+5=15).  The number of hyper-volumes would be the number of volumes plus the number of hyper-volumes in the pyramid of the previous space (5+1).
HyperspaceHypercubeCross Polytope
last revised: 9/27/02