Although it is known for nearly a century that twinning occurs in (bulk) antimony, crystallographic details of the twin domain structure and the origin of the twinning are still lacking to date. Twins in materials are closely related to their mechanical properties, for example, deformation twins. Here, we try to understand the origin of twins in Sb and the relationship between the mechanical properties and different orientations of the twins. The twins' crystallographic orientation and misorientation angle were characterized by Electron Backscatter Diffraction (EBSD), and the atomic structures of twin boundaries were resolved by (scanning) transmission electron microscope ((S)TEM). STEM (HAADF) experiments show that the Sb lattice is rhombohedrally distorted with respect to an underlying cubic structure and that the rhombohedral angle is 86.5 degrees, where the twin boundary is of type (220) when referred to face-centered cubic lattice. Also, the critical finding is that the observation in the atomic STEM images and corresponding diffraction patterns is that the tilt between the (002) planes across the boundary is only about 7 degrees, which can explain why these boundaries are low in energy and so abundant in Sb. We compare the present twinning in Sb with similar twinning observed in GeTe and LaAlO3. Furthermore, we plan to perform density-function theory (DFT) calculations to confirm the low energy structure of the twin boundaries observed in our experimental work.