  The charge density wave (CDW) state of 2H-NbSe$_2$ features commensurate
domains separated by domain boundaries accompanied by phase slips known as
discommensurations. We have unambiguously visualized the structure of CDW
domains using a displacement-field measurement algorithm on a scanning
tunneling microscopy image. Each CDW domain is delimited by three vertices and
three edges of discommensurations and is designated by a triplet of integers
whose sum identifies the types of commensurate structure. The observed
structure is consistent with the alternating triangular tiling pattern
predicted by a phenomenological Landau theory. The domain shape is affected by
crystal defects and also by topological defects in the CDW phase factor. Our
results provide a foundation for a complete understanding of the CDW state and
its relation to the superconducting state.
