The appearance of the sulcus of many monocot pollen grains and the shape of the grains itself varies considerably with the respective pollen configuration (hydrodynamic state): (a) The turgescent, rounded pollen state within the not yet dehisced pollen sac, (b) the dry, contracted state after pollen sac dehiscence and during pollen transport, and (c) the expanded pollen state after (re-) hydration e.g. on the stigma. The sulcus shows diversity of characters, which can, however, be seen only in hydrated pollen. In dry state — and also after acetolysis — the sulcus is mostly furrow-like infolded, and various characters are not longer discernible. Based on investigations of fully hydrated and well prepared pollen, some sulcus forms are described; they are distinguished by their respective shape and surface conditions. In the past, these different sulcus configurations and forms were often overlooked. The sulcus forms are called types for practical purposes only, although there are overlaps between the extremes. The sulcus types may, nevertheless, be important taxonomic characters in some families.