  We report high resolution ARPES measurements of detwinned FeSe single
crystals. The application of a mechanical strain is used to promote the volume
fraction of one of the orthorhombic domains in the sample, which we estimate to
be 80$\%$ detwinned. While the full structure of the electron pockets
consisting of two crossed ellipses may be observed in the tetragonal phase at
temperatures above 90~K, we find that remarkably, only one peanut-shaped
electron pocket oriented along the longer $a$ axis contributes to the ARPES
measurement at low temperatures in the nematic phase, with the expected pocket
along $b$ being not observed. Thus the low temperature Fermi surface of FeSe as
experimentally determined by ARPES consists of one elliptical hole pocket and
one orthogonally-oriented peanut-shaped electron pocket. Our measurements
clarify the long-standing controversies over the interpretation of ARPES
measurements of FeSe.

Superconductivity;Strongly Correlated Electrons