Porosity in the soil forms, except in a few cases, such as vesicles, a continuum. As this is practically impossible to describe, Brewer (1964) classified voids according to their morphology, considering them as individuals. His typology was taken over by Bullock et al. (1985) with some small adaptations. Recently Moran et al. (1988) coined the term "poroids" to indicate the image of pores in a thin section, referring thus to a two-dimensional unit. This term will be also used in this key, as it is very important not to confuse 2-D and 3-D shape descriptions. As different types of voids may show similar poroids in thin sections, this means that a same item keys out several times. Voids between finer constituents, e.g. clay particles, are beyond the resolution of the combination optical microscope/thin section and therefore not taken into account.

1. Poroids with a circular shape

1.1. Circular poroids with very regular smooth walls, often occurring in groups, but not interconnected, frequently with a subhorizontal distribution, and probably derived from spheres in 3-D: vesicles

1.2. Other circular poroids, probably derived by cutting a tubular 3-D shape normal to its axis: channels

2. Poroids with elliptical shape, probably derived by cutting a tubular 3-D shape at an angle to its axis: channels

3. Poroids with elongate shape, with approximately parallel sides:

3.1. Elongate poroids with smooth walls and rounded ends, often curved, derived by cutting a tubular 3-D shape at an acute angle to its axis: channels

3.2. Other elongate poroids, with sharp, pointed ends, often intersecting and with approximately accommodating opposite edges and derived from sheet-like structures in 3-D: planes

4. Irregular shaped poroids

4.1. Irregularly shaped poroids between random packed constituents, which connect to a greater or lesser degree in 2-D: packing voids

4.1.1. packing voids between (coarser) basic constituents only: simple packing voids

4.1.2. other packing voids between aggregates only: compound packing voids

4.1.3. other packing voids: complex packing voids

4.2. Irregularly shaped poroids which do not interconnect in 2-D and probably do not in 3-D.

4.2.1. Non-connecting, irregular poroids which are roughly equant and have smooth walls, often altered by soil fauna: chambers

4.2.2. Non-connecting, irregular poroids which are roughly polygonal (mostly triangular or rectangular) with concave walls occurring between welded aggregates star-like vughs

4.2.3. Other non-connecting, irregular poroids vughs non-connecting, except for small desiccation planes due to preparation.