The (inter-)dental non-sibilant fricatives, consonants articulated with the tongue tip or blade against or between the front teeth, are rare among the world’s languages but, nevertheless, are present in the sound inventories of some of the most spoken languages in existence. Here we try to shed light on the reason(s) for their distribution using multiple approaches, ranging from examining large cross-linguistic databases and phylogenetic reconstructions to the analysis of speech production data and anatomical measurements of rigid oral cavity structures obtained using intraoral 3D optical scanning. With these, we don’t only confirm that dental fricatives are rare among the present-day languages, but also that they have likely been so as far back as language families can be reconstructed, and that they are rarely borrowed between languages. The experimental data from L2 English speakers seem to suggest that details of the anatomy of the anterior vocal tract may play a role in the success of their acquisition. Therefore, dental fricatives are rare speech sounds for a multitude of reasons touching upon their articulation, acoustics, and confusability with other speech sounds, including the difficulty to produce in both L1 and L2 acquisition, the difficulty to perceive in L2 and in borrowing situations, and the rarity of attested sound changes producing them. Nevertheless, their frequent loss and merging with other phonemes in language change. Moreover, our data suggest that tiny, continuous, and overlapping patterns of variation in the anatomy of the anterior oral vocal tract may help explain their instability and geographic patterning.