Definition
Triangular prism
A triangular prism is a solid with two congruent, parallel triangular bases joined by three faces.
What it means
Imagine copying a triangle without turning it and sliding the copy straight back. Join matching corners. The two triangles are the bases. The finished solid has 5 faces, 9 edges, and 6 vertices, whichever way it is turned.
Remember it
Copy a triangle straight back, then join matching corners.
Example
Start with two matching triangles. They contribute 6 vertices and 6 triangle edges. Join the three matching vertex pairs to add 3 more edges. The result has 6 vertices, 9 edges, and 5 faces.
Watch for
A triangular pyramid is not a triangular prism: it has one base and its other triangular faces meet at one apex, instead of having two separate matching triangle bases.
Common mix-up
The word “triangular” names the prism’s two bases, not every face. Its other three faces are parallelograms, often drawn as rectangles.
Exact meaning
A triangular prism is a prism whose two bases are congruent, parallel triangles. Joining corresponding sides of those triangles forms its three other faces.