This specification covers the materials, sizing, and manufacturing details of test headforms for use in the testing of protective headgear. This specification identifies the requirements for five test headform size codes: A, E, J, M, and O. All datum levels shall be quoted relative to the reference plane. The headforms shall be symmetrical about the midsagittal plane. Internal geometry may be defined to satisfy the requirements of the individual test specifications. The impact headforms shall be made of K1A-F magnesium, with the prescribed zirconium and magnesium composition, and shall conform to the specified requirements for: external dimensions, performance (resonant frequency), mass, and center of gravity. All impact headforms shall be marked with: headform size, basic plane, reference plane, and midsagittal (longitudinal) and coronal (transverse) planes through the vertical axis. Other headforms shall be made of material of sufficient strength and stiffness to maintain their geometry during testing and shall comply to the external dimension requirements.