While performing prism and cover measurements for strabismus with the head tilted, prisms held before the eye should be positioned so that the base of the "horizontal" prism is parallel to the lateral wall of the orbit, and the base of the "vertical" prism is parallel to the floor of the orbit. This placement is required because with the head tilted, torsional compensation of the globe is minimal.
Helveston EM. Prism PlacementMeasurements of Horizontal and Vertical Deviations With the Head Tilted. Arch Ophthalmol. 1975;93(7):483-486. doi:10.1001/archopht.1975.01010020499003