T. tenuissimum is well-defined in its centre of distribution in the Kalahari, differing from the other smooth-seeded members of the T. caffrum complex by the combination of linear revolute leaves, filiform peduncles, much smaller flowers with relatively few stamens, and a strongly compressed comma-shaped seed.Herbarium material can be difficult to distinguish from narrower-leaved forms of T. arnotii, when petal size and stamen number are not easily determined in the fugacious flowers.East African specimens, at the northernmost limit of its range, usually have more stamens, plumper, more lenticular seeds and sometimes broader leaves, probably due to introgression from the commoner T. arnotii. T. tenuissimum is sometimes confused with the southern African species T. crispatulum Dinter (see under T. arnotii). T. tenuissimum is easily separated from this by its linear leaves with revolute margins, non-papillose stems, and fewer stamens.