Which of the following types of tapered threaded fittings is not included in the standard categories?

The type of tapered threaded fitting that is not included in the standard categories is brass. In the context of piping and fittings, the standard categories typically encompass materials like malleable iron, forged steel, and cast iron, which are commonly used in various industrial applications due to their mechanical properties and suitability for high-pressure environments.

Brass fittings, while they do exist, are usually categorized differently because they are often associated with plumbing and low-pressure applications. Brass is an alloy of copper and zinc and is primarily used for its corrosion resistance and aesthetic qualities rather than the robust performance required in high-pressure systems where malleable, forged, or cast fittings would be more appropriate. This distinction is essential when understanding the classifications of fittings in piping systems and their intended applications.