Seen edge-on, the fundamental plane (FP) of elliptical galaxies is very thin and tilted, which corresponds to a very small dispersion (∼12%) of the M/L ratio at each location on the FP, and to a trend by a factor ∼3 of M/L along the FP, respectively. In this Letter we investigate two possible origins of the tilt: a systematic change of the stellar initial mass function (IMF), and a trend in the relative distributions of the bright and dark matter. In both cases we find that major changes in either the IMF or the bright/dark matter relative distributions are required along the FP to account for its tilt, while its constant thickness requires an extremely small dispersion in the IMF or in the bright/dark matter distribution at every location on the FP