The tetragonal crystal system has three axes of symmetry: two axes, of equal length, lie in a plane at 90°; the third is longer or shorter and is at right angles to the others.
Crystals are often long and slender or even needle-like; characteristic forms are four-sided prisms, pyramids, and dipyramids.
- Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987). pp. 33-34