"What Shape Is the Nut on a Fire Hydrant, and Why?"

photo of hydrant with pentagonal prism nuts

This question is frequently asked, and there is more than one answer.

In the United States, the most common shape is that of a pentagonal prism, as shown in the drawing on the right. The faces that define this polyhedron are pentagons. The 5 sided characteristic of the nut was chosen to offer a measure of tamper resistance. Hydrants can be dangerous, and only firefighters and other authorized persons should open a fire hydrant.

pentagonal prismpentagonal prism

Sometimes the 5-sided nut has a slight taper. The shape of the nut is then called a truncated pentagonal pyramid. The pentagon face on the outside is called the minor base, and is smaller than the pentagon where the nut is is attached to the hydrant. The larger pentagon where the nut is attached to the hydrant is called the major base.

truncated pentagonal pyramid

truncated pentagonal pyramid

photo of tetragonal prism hydrant nut

In Canada and Japan, the most common shape for a fire hydrant's operating nut is a tetragonal prism, as illustrated in the photo on the left.
The polygon that defines this prism is the square.

tetragonal prismtetragonal prism

The tetragonal prism is also found on hydrants in some communities in the USA.

truncated tetragonal pyramid triangular prism hexagonal prism octagonal prism

The shapes illustrated above are not the only ones found on hydrants. Other shapes for the hydrant nut include:

octagonal prism
hexagonal prism
triangular prism
truncated tetragonal pyramid

Can you match these names with the drawings at left?

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