  Electron-hole (e-h) compensation is a hallmark of multi-band semimetals with
extremely large magnetoresistance (XMR) and has been considered to be the basis
for XMR. Recent spectroscopic experiments, however, reveal that YSb with
non-saturating magnetoresistance is uncompensated, questioning the e-h
compensation scenario for XMR. Here we demonstrate with magnetoresistivity and
angle dependent Shubnikov - de Haas (SdH) quantum oscillation measurements that
YSb does have nearly perfect e-h compensation, with a density ratio of $0.95$
for electrons and holes. The density and mobility anisotropy of the charge
carriers revealed in the SdH experiments allow us to quantitatively describe
the magnetoresistance with an anisotropic multi-band model that includes
contributions from all Fermi pockets. We elucidate the role of compensated
multi-bands in the occurrence of XMR by demonstrating the evolution of
calculated magnetoresistances for a single band and for various combinations of
electron and hole Fermi pockets.
