Figure 2b. The vertical rod (bold line segment) as observed in its own inertial frame of reference. The rod’s proper length is equal to the unit length, as defined in the previous figure

From

Why and How Special Relativity Fails in the Second Spatial Dimension? An Introduction to the Preferred Frame Theory

Maciej Rybicki

International Journal of Physics. 2024, 12(4), 175-195 doi:10.12691/ijp-12-4-5